It's been a rare occasion when we've been at an RV park and *haven't* been assigned a site. There have been a few times when we've gone back to the office to say that the site won't work for us.
In campgrounds, it's another thing entirely, since I can't remember a time when we've been assigned a site. In these cases, we look for a site where our satellite TV dish isn't blocked by trees (we have a dome on the roof at the back of our rig and don't carry a portable dish), one that is relatively level and wide enough to easily deploy our 3 slides. Also, if we're going to be dry camping, we want a site where the sun will shine on our solar panels for most of the day (we think trees are highly overrated, particularly in dry camping situations