Tube Amp out of Bias-what does it sound like?

Proper biasing can make a sonic difference. As little as a 5ma difference lower than optimum can change my sound from neutral/warm to neutral/cool. 5ma over optimum bias can shorten tube life quite a bit. Bais requirements change as the tubes age and, depending on use and tube types, should probably be checked at least every 6 months (I check mine every 2 months). Checking bias will also give you advance warning of an aging tube - you can change it before it dies or shorts out. The latter is well worth avoiding.

In a nutshell, if they are biased too low the sound will be groggy and not very involving, too high will sound better but you will put additional wear on your tubes.

I agree with what Newbee and Philojet have said.
Out of bias does affect the sound. That being said, it is unwise to say to low or to high. To much room for confusion. Much more appropriate to say to cold or to hot. To cold there is not enough current in the output stage to work correctly. Sound will be buzzy, not clean etc.. To hot can sound very good (unless way to hot) but lots of drawbacks. Tubes wear out much to quickly, the whole system is stressed to much which isnt good.

How long should you wait after turn to check/adjust bias?