You do not "have to use a hitch extension".
I hate those things, they make all the geometry wrong. I opted to extend my trailer tongue instead. It tows a LOT better that way and I consider it a safer way to go. The further behind your rear axle the hitch is, the more leverage your trailer has against your truck and the more it unloads your front tires.
Stretching the trailer tongue effectively increases the trailer's length and that in itself also helps it tow straighter. Everything is good about doing it this way and everything is bad about extending the hitch. I don't understand why I seem to be the only person who does it!