Some ladies do like to be absolutely sure, so they base their scalp the day before AND the day of a relaxer.
I'm starting to think that what you base your scalp with is more important than how many times you base it.
Vaseline ain't cuttin it for me anymore When I self-relaxed, my head was crispier than Fire Marshal Bill
My old hairdresser used to use something thick and petroleum-based to base my scalp. It came in an applicator bottle, but for the life of me, I can't remember what the brand name was
When I've been scratching, plain baby oil moderately applied the day/night before works wonders. When I do that I don't have any burning (but my stylist always bases well.