Warm Tubes

I've been reading up on tube pre amps and in particular the ConradJohnsons. In different posts prople have mentioned that the keep their pre on all the time or 1 hour warm-ups, etc. Never have been around tubes audio so evn though it seems obvious it still suprised me. No just flipping on the system. Is a system warm up of an hour or more the rule? Any other nuances with tubed equipment I should be aware of? Thanks, Dave