[Comattack]

I've always opened the bleed valve to compress the pistons, then topped of the brake fluid. (to make up for fluid pushed out) If you do brake front an rear at the same time, topping of isn't such a bad idea.

If you keep contant pressure on the piston before you open the bleed valve, and then close the valve just before fluid competely stops flowing, no air will get in and you won't have to bleed the lines. The same is true if you are going to change fluid and bleed them anyway. You still want to keep as much air out as possible.