Biasing tubes/tube gear

Is it necessary to bias tubes/tube gear on any kind of regular basis or just upon installation?
DEpends on amp. If it is self bias no problem, if not you need to bias the power tubes eveytime you change.
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.
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.
Catama, that would be wrong, a fixed bias amp has pots to adjust bias and should be checked periodically. An auto bias, or cathode bias, amp has no such controls and cannot be biased by the user.