The farther the ball is BEHIND the axle, the LONGER the lever arm. That is, the farther the ball is behind the axle, the more LEVERAGE the trailer has over the tow vehicle.
(That leverage can be countered somewhat by a longer wheelbase on the tow vehicle, and by a shorter rear overhang on the trailer.)
Fivers are less prone to sway than travel trailers (of similar size and proportion) because of the shorter lever arm. That said though, sway doesn't seem to be quite the same problem for our little trailers as it is for the big boys. The exception might be if you tried to tow with a car (like an old Jeep, for instance) that had a very short wheelbase.
In adition to sway, you must consider the effects of weight
distribution when deciding where to put the hitch. Putting the kingpin weight
far behind the axle might may take weight
off the front and adversly affect both how the headlights point and and how the truck brakes
5ers all come with a sort of "kingpin extension" it may LOOK (in pictures) as though the ball is further back than it really is. If you have about 60 inches from the center of your axle to your back bumper, you should be able to mount the hitch right over the axle.
After I picked up my fiver at the factory in April, I drove across Iowa in some pretty high winds. But I didn't have a clue that it was windy until I got out of my truck and was about blown away. I talked to a guy in the campground in Iowa who had had some problems with his stick and tin 27 foot travel trailer. He said, enviously, "I bet you can't even feel the wind!"