If you read Randy Ulmer's tips on accuracy, his one comment is that you should use a peep for the ultimate in accuracy. The kisser is a benefit too if you do not have a solid anchor point. If you do have a solid anchor then a kisser is not really necessary.
I used to use both but since I've been shooting more and more, I find the kisser not to be necessary. My anchor is the back of my hand behind my jaw. I've used this anchor so often that I can draw with my eyes closed and when I open them my eye is dead center in line with my peep.