It's actually the tubes that vary most, now. Most modern amps have tighter tolerance components that dont drift much. In fact, many players will find they prefer different bias than one another, due to their ear, and play style. Now, that doesn't mean we should get careless, but these aren't Hi Fi amps, we dont need the bias within a specific spot, within a 1% tolerance to have an amp that sounds and functions properly. I usually dont bias my power tubes at max safe dissipation, so I usually dont need to worry about new tubes red plating, or overtaxing the transformer.

There's no hard fast rule, as long as you're in a safe operating range. The power tubes have a pretty wide range of safe operating bias range, and certain (brand) tubes sound better/worse at certain bias levels running hotter/colder. It's always good to check readings, but I have swapped power tubes numerous times on Fixed adjustable bias, without tuning. IME, that's also usually the case with fixed bias amps.

Cathode biasing requires no tuning unless components have drifted.

Self bias guitar amp