First, if we do WDW first, I do all clothes before leaving WDW (that way, I'm starting with totally clean stuff for cruise.) I also pack a "cruise only" suitcase with our formals and other stuff we won't need on land. We usually do the 7 day Magic cruise with 4-5 days at the parks first.....
Then for laundry....I come back from Castaway about an hour before we have to be on board. That way, I've always been able to run our suits thru the washer and not have to take them home wet and nasty...ditto cover-ups, etc. in that load. For other laundry if needed, do it on land days. Maybe I'm rude, but if people have left their stuff in washer or dryer after load is done, I'll wait about 5 minutes, then unload their stuff onto the metal table that is in the laundry room. There are clearly timers on both washers and dryers, and thoughtful people check the timers and come back ON TIME to get their stuff and free up machines for the next user. I've even had people offer me their "unused" time on the dryers. I always bring my soap from home (cheaper and easier)....most people are very nice.
It's not too expensive to have a load of laundry done for you (well, compared to a cruise cost!), but I've not had trouble getting a machine on 10 Magic cruises....sometimes had to wait 30 min or so...