Knowing the tempo of the sample is important (IMO). Especially when you want to work with bigger sized chops. If you work with smaller chuncks it won't matter as much.
I personally chop however I feel like at the moment. Sometimes I chop exact phrases of the sample (a chord, a vocal part/word, a specific note) other times I chop in smaller 1/8 pieces so I can go crazier. It will depend on what you can get out of the sample and how busy it is really.
Do you have any tracks to share? I'm curious to hear what you have made so far.