Buttered coffee won't take your body out of ketosis, so it's not really cheating.

If I ate earlier the night before (6p-ish), and I know I'm going to be out several hours playing/teaching tennis and not back until maybe 1-2p, I'll do a bulletproof to tide me over. The key is to be flexible so that your body is always adapting and is comfortable switching modes between burning fat, ketones...or even carbs.