or while squeezing the pads apart, pull out the bleed plug in the lever and allow some fluid out (just a little)
I wouldn't do that, you'll just end up needing to bleed them.
I used to use the following procedure:
1. Old pads out, clean out obvious gunk.
2. Put the shaft of a small spanner (8mm?) in the caliper and pump the level until the centre pin of both pistons meets it.
3. Lube the pistons with some silicone spray/grease.
4. Press the pistons back using a 10/12mm spanner.
5. Repeat 2-4 depending on how freely the pistons are moving.
6. Clean off any lingering lube to avoid contaminating the pads.
7. Put the new pads in.
If the pistons still creep out and won't stay fully retracted then with the new pads still in wedge them apart using old business cards and give the lever a few really hard pulls.