Helper springs or air bags?

Slight sag is normal. Most trucks have the rears higher so when loaded they're level or so with the front. You wouldn't want the truck level to start with and load it up. If so the headlights would point up to high.
If it tows fine then leave it.

FWIW the front is more critical. Set the front properly and the rear is what it is. You don't need the rear to be at the OEM height when hitched.