A rented gun should be about as accurate as it can be depending on how well the rental gun has been maintained. If the range is worth going to at all they should be taking care of the guns they rent. It's just good business. Now guns with an adjustable site might fall out of true a little easier than some with fixed sights but you should still be able to get a good idea of how well it works. Most all range rentals I've seen have fixed sights so it shouldn't me much of an issue keeping them sighted in.
The biggest thing you are going to want to look at with a rental is how it fits, how the felt recoil feels to you and weather or not you can deal with it. Sights can be replaces or set to be best for you later. some guns liek different ammo so you might not get the best idea how something shoots until you put a few different rounds brands through it. It might shoots pretty straight but you might not like the sights the gun has but if you actually bought one and put a set of sights you liked more it may get a lot more accurate. So take shooting rentals with a grain of salt but you should get a pretty good idea shooting a rental if it is one that you would have a good time shooting well.