I know of no reason why you would ever crack a bleeder valve for anything other than bleeding. PEOPLE, PLEASE DO NOT DO THIS
! In doing so you are breaching a high pressure hydraulic system, and that's bad. Sorry to shout, but I've read a few posts here regarding this practice and it bothers me. I used to work on helicopters, and such a procedure could have you falling out of the sky. In the first place, your pistons should compress fairly easily; if not, your technique needs a little work. I either use a Channel Lock pliers to squeeze the old pad against the piston(s), or remove just one old pad and use the rubberized end of that same ChannelLock to lever against the piston (no damage to the rotor that way). Doing a pad change, you should never have to add fluid, since you're pushing quite a bit of fluid back into the master cylinder...it's more of a concern that you don't overflow the master.
You need to bleed your system, unquestionably. When you cracked the bleeder, you likely let a bit of air in. And frankly if the system DID ingest some air, it may have also ingested any corrosion or dirt particulates that were in/around your bleeder valve when you cracked it open(more of a concern for older cars). Get the air out of there and you'll be fine