Your ears will tell you all you need to know. I will tune my entire kit every time I hit a new venue, just to match it to the room. If something drops tuning whilst I am playing, I'll rectify it on the fly. Tuning when changing venue (gigs, practices etc) is always good, as what sound sweet where you play at home (which could be small and dead) could be very different to, say, a large pub (big and bright). Snares especially will be very sensitive to this.
As a rule of thumb, if you think you need to ask this question, it's probably time...