This is a good point, and it's worth noting that even making a reservation in advance might not guarantee what you want. When I was moving, Hertz accidentally rented the van my father and I reserved in advance to someone else. Ultimately they gave us a similar van and there were no major issues, but this still left me feeling uneasy.
I imagine that renting via a different service like Zipcar wouldn't have this issue, though I haven't used Zipcar yet.
ZIpcar has a different set of problems; there is probably a car of the specific type you want available, but getting to it (and from it) may not be convenient, because it has a specific place in the city that it lives. ANd if that's 30 minutes away by your fastest non-car transportation, that's pretty frustrating.
I suppose for moving etc., you could call a cab to get you from old home to the Zipcar, and then again from the Zipcar parking to new home. That feels strange and probably-inefficient, but I don't have evidence to back up that feeling.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.