I don't use a G clamp, never needed such pressure, just use a long screwdriver well placed to GENTLY push the pistons back in, i've been doing it this way for more years than i care to remember and not had a master cyl problem yet.

It certainly wouldn't hurt to do it, the correct way is as you state, and yes i would use that opportunity to give the brakes a bleed and therefore an early fluid change, one benefit of this is that there is less chance of the bleed screws seizing up when they do need to be undone.

I've always clamped the pipe and opened the bleed nipple. I've also always bled the brakes whenever changing pads. It's the fluid in the pipe that will be most contaminated, especially with water vapour which passes through hoses (though less so these days with improvements in materials).