Though not a common problem to see with these transmissions it is one that can occur. There are not many components in the reverse system which rules out a lot of variables. The problems found that can cause reverse problems are - Poor sealing lip seal on the reverse apply servo, Worn reverse boost valve and sleeve, Partially blown channel plate gasket, overall low line pressure condition. These transmissions from the factory use a short design lip seal on the reverse servo. The aftermarket seal and gasket kits as well as Transgo include a long design lip seal which aids in some reverse problems, especially temperature related.
On a fresh rebuild- If you have no reverse or severe slippage then it is possible the reverse servo pin is not properly lined up with the band. It is possible to check this with the transmission installed in the vehicle but on some models you may have to remove the rear exhaust manifold to gain access. This is an important step that can be overlooked during a rebuild and since the transmission is likely being built sideways it can fall out of place. Once installed in the vehicle you can remove the reverse servo and the band may drop into position ok on its own- and simply just lifting up and dropping the servo back down will properly align it.