I have a slightly different take on things, but won't argue, since there is no absolute right or wrong in casting. If your method gives you good results, then that is all that matters.
I do have a hunch that your dependence on high temperatures and tin is a symptom of poor pouring technique. The key to getting good fill with straight WW and modest temperatures is FLOW
Another obvious, but often overlooked tip, is don't expect good fill on the first few pours with straight WW. It sometimes happens, but is the exception, not the rule. I have molds that even after being preheated to near optimal temperature, still require 30 minutes of casting before they start behaving themselves.
As for cooling the mold between pours with a damp rag or such, that would actually slow me down, not speed things up (exception being the very heaviest bullets, like a 2-cavity 700 grainer that does indeed require a cooling period between pours). With saner bullet weights and modest casting temps, I can pour immediately after dumping the bullets, without ill effects like smearing the sprue.
As bullet weight creeps up, it may be necessary to pause for a few seconds before pouring again. Meaning paused with the empty mold in the open position. That's how I regulate mold temperature. The thermostat setting on my WW pot never changes.
The cooler you cast, the less time it takes for the sprue to harden, and the higher your production rate, without pausing to mess around with damp rags and other gimmicks. That's the beauty of low-temperature casting. The trick to making it work is FLOW
If you are casting frosty bullets with a tin/antimony alloy, I can almost guarantee there will be significant shrinkage. Yet most people never notice. Why ? Because they don't know how to measure bullets. Don't measure the entire bullet -- that only picks up the high spots and misses the shrunken spots. Measure the bottom band alone, in 3 spots. If the bullet has a check shank, measure that, too, again in 3 spots. The worst shrinkage typically takes place on the bottom band and check shank.
Bullets with shrunken bottom bands may, in fact, shoot well enough. The front bands seal the bore, the bottom bands will obturate, so all is well, and the average shooter may never have reason to complain.
An acquaintance had a custom Lee 6 cavity made to his design, and spec'd to 0.360". And we all know that Lee guarantees 0.001" tolerance, ha ha ha.
He gave me some sample bullets, which I promptly measured, and found that while some of the bands on some of the bullets did measure 0.360", others measured as small as 0.355". Yet my friend was happy with the mold, and said they shot well. My guess is those bullets were heat shrinking, which is no fault of Lee, just a fact of life.
Note that I test every mold I make, without using smoke or Bull-plate or any other gimmicks on the cavities. As long as the design choices are decent, I've always been able to get the mold to fill out with straight WW at 675 - 725 degrees. Exceptions are a bullet that is too light for the block, or some bullets that are just very difficult to fill by design, though the current program eliminates the worst of those.
No, straight WW does not fill out sharp edges as crisply as other alloys, but it shoots fine, and that's all that matters.
For those that apply some sort of mold prep to the cavities, be aware that it does change the diameter, and does impair mold alignment. You may not be able to measure a difference (see previous comment about measuring bullets) but it's there.
I'm not opposed to smoking aluminum cavities, used to do it myself to Lee, NEI, and LBT molds. However, my casting techniques have improved to where I no longer find it necessary to rely on gimmicks like smoking cavities.
Finally, after taking great pains to make a perfect mold, and to use a perfect casting technique, Mr. Murphy throws us a curve ball. It is a rare casting session where the faces of the mold do not acquire some lead splatter. This prevents the mold from closing completely, and throws off diameters and alignment. I'm constantly looking for lead spatter, and will wipe it off gently with fine steel wool, but despite my best efforts, inevitably I'll end up with a few oversize bullets due to lead splatter. Is this a problem ? Only if it creates problems with a bore riding nose, or if it creates problems with gas check fit. Otherwise, the sizing die makes everything right. A good sizing die is the cast bullet shooter's best friend.
I would agree that it is fine to have a bullet drop out 0.001" over the size-to diameter. However, I often experiment with different size-to diameters before I settle on the best diameter for that particular gun/bullet/load combination. An oversize bullet gives me room to experiment, a minimal diameter bullet does not.
Or you may buy a mold for a gun with tight dimensions, only to later acquire another gun in the same caliber except with sloppy dimensions. If your mold was sized minimally for the tight gun, now you have to buy a new mold for the sloppy gun. On the other hand, an oversized bullet for the sloppy gun can always be sized down to fit the tight gun. A good sizing die is the cast bullet shooter's best friend.
Or if you experience heat shrink, which is to a degree unavoidable with some molds, an oversize plain base bullet gives you more wiggle room to compensate for heat shrink.
On a typical big bore mold, I can observe 0.002" diameter variation from one pour to the next, without really trying, due solely to slight changes in mold temperature and pouring technique. Yes, if you are a good caster, and if you make an effort to use a consistent technique, and if you don't pick up any lead splatter, then you can produce very consistant diameters. But not everyone is a good caster, and not everyone uses the same technique.
My point being, your technique may result in a 0.002" difference compared to my technique. The idea that a mold maker can guarantee very tight as-cast tolerances is laughable. I used to be surprised when customers got different diameters than me. Now I am surprised when they get the same diameters !
If I were to guarantee the MOLD
diameter, rather than the as-cast bullet diameter, my job would be ever so much easier, and I could brag about tighter tolerances. I could probably double my production and my income. Machining to precise tolerances is a science. Getting a gravity pour mold to fill out consistently is a highly imperfect art.