I was wondering when you found the time to write... That was quick.
I have a routine that "compensates" for these issues fairly well, but not perfectly. And you're right, the reason it probably works fairly consistently for me is that I allow warmup as part of the routine.
If you are using a regular Tungsten light source, you probably have no issues significant to worry about. Ned and I have flourescent lights that really do brighten significantly in the first few seconds if you turn them on when they're cold.