Well, that's what the name says - pre-workout. Therefore, it is best to use them only before working out, as they are supposed to give you an edge during training, and it is quite pointless to use pre-workouts on rest days. Therefore, just use it before working out. Personally, I would recommend DSpark as the optimal preworkout - it has no stims in it, and yet it will give some amazing pumps and energy in the gym. Try to avoid pre-workouts with stims.