Biasing tubes/tube gear

DEpends on amp. If it is self bias no problem, if not you need to bias the power tubes eveytime you change.

I think it's a good idea to periodically check tube bias to ensure that the tubes still function. Further, as Ryriken says, if a tube amp does not self-bias, occasional adjustments are required. Maybe because I'm OCD or just bored, I check tube bias every 2 or 3 weeks just because .... More frequently if I hear distortion.

Whenever you change the power tubes and every year to make sure it isn't off. If your amp is auto biasing or fixed bias, then you don't have to.