Question on biasing an amplifier

When you are about to check the bias on a tubed amp with a hand held digital voltage meter
Should the amp be off before inserting the leads of the meter
or is it safe to insert the leads with the amp on and playing
Thank you
Hope to its safe when the amp is on; it is what I have always done.
I plug in my multimeter while the amp is on. It's safe. If your multimeter has a loose lead, however, you'll get a snap-crackle-pop through your speakers. That will definitely shake you up, but it won't hurt you.
Actually, I don't think taking a reading from off to on would make sense, since it takes a good 15 minutes or so for the bias to stabilize.