I also used to apply grease on cotters way back in the '70-ies.
One tap used to be enough to get the (greased) cotter back out. If you unscrew the nut just far enough and let it sit on the threads it will not deform nor be damaged by one (gentle) tap.
BTW do not hit the cotters without supporting the BB axl. The bearings and races do not like a violent approach.