Center of hitch should be over axle. Some prefer an inch or two in front of axle which is OK. If the center of the hitch is behind the axle, you start taking weight off of the front axles which isn't good. This affects handling and could contribute to an overweight condition on the rear axle.
Mine is directly over the axle. Coincidently when I weighed the truck with and without the trailer, the weight on the front axle was the same.
I did tow my fiver 20 miles once with the sliding hitch set back by mistake. I couldn't tell and difference in handling but I wouldn't recommend it.