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.
Biasing tubes/tube gear
Is it necessary to bias tubes/tube gear on any kind of regular basis or just upon installation?