Over and Under Biasing as means for tone control

What is the word about using bias as tone control?

First what is the risk to Tubes and equipment by doing that?
Second what is the actual effect of doing that as far as tone?

I know in the Guitar world it is not uncommon to do, but Guitar Amps are going for shades of distortion, unlike HiFi listening amps.

I would be interested to hear what people think.
Thanks!