Tubes will have a specified voltage that you shouldn't exceed. This will vary with the specific tube and isn't a one size fits all for every 6L6, EL34, etc. If you know the bias voltage of your amp, simply make certain the tubes you buy can withstand the voltage you want to run through them. Some tubes don't come alive until they are run a bit hot. Anyone selling NOS tubes should be able to provide the information you need to make a safe decision. If they can't, shop elsewhere. The amp will specify what the voltage drop should be to set correct bias levels. You can run the tubes a bit hot and gain a few watts and a bit more class A operation from the amp, while cutting down on tube life. Or, you can run the tubes cooler and extend life and change the sound slightly. I would start by setting the tubes to the factory spec and then deciding whether and how to change from there. Raising the voltage will also stress other components beyond the tubes and you do risk damaging the amp if you push too hard. After you've set the bias, turn the room lights out and make sure all the tubes are glowing at an equal intensity and never a cherry red.