Rover, generally speaking on revolvers you size to the cylinder throat, assuming the throat is larger than the groove diameter. If reversed that is a different story for a different thread. Your Ruger's should be fine with .429.
Rifles should have the largest diameter that will RELIBLY chamber in a fouled gun, some 44-40 will chamber .430 when clean but not after a couple stages. In this case drop down to .429. Or just start with 429 in the beginning unless there is a problem.
Sizing any lubed bullet down 2/1000 is no problem.
Shooting undersize bullets with lube is a leading issue looking for a place to happen. Soft bullets like ours should be at least groove diameter or larger when they leave the cylinder. Shooting undersized coated should not lead unless the coating is broken. Accuracy may suffer, but probably not enough to matter at average SASS distance.
Sources? outlawbullets.com
I can't speak to coated bullets from personal experience but sizing down a 1000 or two shouldn't be a problem either.
T Bone