The problem with backing up a car behind anything is the tendency of the front wheels to caster, or turn to the limit on one side or the other. If the front steering is locked straight, an experienced person should be able to back one. What actually happens is that the wheels turn hard to one side, and you can severly damage either the steering or the tow bar and brackets, or both. Most people have backed one up for a short distance, especially if taking it very slow, and generally in a straight line.
So the correct answer is anytime you want to back up! If we are going to a campground, one of us will go in and register or whatever, and the other will disconnect the toad. Its not a big thing, usually less than a minute with our Blue Ox towbar and brackets. Generally the rule is if the toad is hooked up, you don't have reverse.