I think location, body of water, etc... are huge factors in whether you need to be worried about your boats aft hanging out past your dock. 20' dock, if mored or secured well, should be more than sturdy enough for your boat.
Whether or not the aft sticking out is a concern is hopefully something you can ascertain. I see the danger at a marina, or canal/river, etc in which people are hugging shoreline or channels more.
However, on my lake, which is very big and shallow for some distance from the shore, you would have no concerns. You wouldn't have anyone really coming within 150' or more feet of you dock/boat. Heck, by ordinance they need to be something like 150-200' out from any dock. Not that everyone sticks to it, but most due, and no one has came close to our dock/boat ever due to the nature of our lake.