I always take the speedo off the tank, and the tank off the bike. I leave the speedo connected to the wires, and lay it up over the bars, or bungee it to the shield to keep it out of the way. As much as paint and tins cost, I feel much better getting the tank clear off, out of the way, and resting on something soft, either on the floor or the bench, where its not going anywhere.
A scratched fuel tank makes for an ugly bike, and you just can't earn back enough "atta boy's" to make up for it.
Besides, with it out of the way, you can take your time cleaning and polishing the rocker covers, and making sure all the grit etc, is out from around the base of the plugs when removing them. JMHO