Tube change

I'm about to change my 6550/12AT7/6350 tubes on my VTL MB250. Do I need to set the bias dial to zero before replace with new tubes?
You don't absolutely have to, but- It won't hurt. I always turn my bias pot down 50% before changing power tubes, then bring the bias up to spec slowly as things warm up(I wait 1/2hr at least for the voltages to settle), then keep an eye on my ammeter for the next couple hours. I check my bias every week(no built-in meters). With the cost of good tubes nowdays, I just feel like taking extra precautions.
I recently discussed this with an extremely competent technician regarding my VTL amps. He said I should change one tube at a time and bias it. Since the bias of each tube affects the others, it would be a real bear to do them all at once.