i mentioned to the guy at my LBS that i'd like a 15T cog for my 42T chainring, and he almost started laughing. he said that if i want to keep my knees healthy, i should go 42x19/42x18, or 42x17 at the very tallest.
mind you, this person has tons of fixie experience -- it's almost all he does in the shop...
so i don't know. i think i'll get the 42x18 and take it from there, moving down as i become at ease with it.