In answer to EustaceSM, in my experience the wardrobe master or costume supervisor usually makes the decision on where we stand going into tech. Often, we'll have a quickchange rehearsal just for the crucial parts...for instance, for My Fair Lady, we did the mass quickchange into Ascot, and then continued through the first underscoring, as we did a fashion parade with changes. We also had a few 10 and 15 second changes from Cockney to Butler or reverse that we practiced before the whole show. Everything else, we felt we could address during the dress rehearsal itself. I mark the quickchanges into my own script so that when we're in tech and I want to call a hold I know if it matters for something going on backstage, too, and will say "we'll be running this through to Eliza's next entrance for the quickchange" if at all possible for such a circumstance.
For stage management, I time every single entrance and exit during runthroughs, so I can give that information to the wardrobe people for their own prep, as well as work on a document in a grid format that shows who is which character (or costume) for each scene. It also lists the location they enter/exit and what page. As for the quickchange itself, I usually trust my wardrobe folks to figure out the best way to do it, as that's more their expertise than mine...though we may have figured some things out in the rehearsal hall already. (For Crowns, we knew they could start changing their gloves offstage first, while still singing the end of a verse with live offstage mics, and when they stopped singing, then they'd attend to their hat and jacket, etc.)
Hope this helps. (I also just noticed this was a merged thread, so be sure to look for other answers "above" that are similar.)
Erin