Most rooms are fine but if you check in too late your chances of getting a "bad" room or one that doesn't not meet your basic requests will be higher.
This is correct. Yes, the available inventory (in terms of "good" vs. "less good" rooms) changes every day depending who is checking out. However, the better space that becomes available on any given day tends to go faster than the less attractive space.
So you are almost always better off checking in early,
and politely telling the front desk CM that you are willing to wait for the villa that comes closest to meeting all your requests. That way, you can avoid the "room ready' disaster (being dumped into whatever is immediately available at the time, which often isn't what you requested).
Instead, the CM will assess the space that will be opening up that day, and will assign you to the best villa (e.g. that which meets or comes as close as possible to meeting your requests). Unless it's immediately available (which outside of very slow periods is unusual),
they won't tell you in advance what the villa # is. Rather, they print out the resort cards for your party (so you can all start enjoying the resort/parks and making charges) and tell you to call back later (usually after 4pm) to get your villa #.
Bottom line: do check in as early as possible, and think of it as grabbing a space toward the front of the request line, not actually getting into your villa early. We've followed this strategy ever since joinng
DVC in 2000, and it's almost always resulted in us getting exactly what we requested. The one time it didn't was at HH during a mid-August check in, we had to request a room move on our second day -- which you are charged for -- to get out of a "dumpster view" type 1 BR.
One last hint: Friday is a great day to check in, since so many DVC members avoid Fri-Sat stays to save points. Ergo, the amount of available "good" space at most DVC resorts increases dramatically on Friday mornings.