Frets wear slowly as you play your banjo. One day you start to notice that your banjo won’t stay in tune when you capo or things just don’t sound quite right. Not that a certain amount of fret wear is bad, but there will come a time when it will be needed. This is a procedure that will extend the life of your frets without having to replace them. This will only prolong a refret or at least a partial refret, but it is well worth looking into. My personal rule of thumb is that if on more than 30% of the original fret height compromised by fret wear, then a level, crown and polish can be done.
The truss rod needs to be adjusted so that the neck is flat and straight. If you can’t get the neck to go flat with the straightedge, you may have other issues. Assuming that your neck works properly, you can move on to leveling the tops of the frets