I always use a strap with the guitar, even sitting down. It's a little more stable.

I would advise, however, not using a strap with the banjo.

If you should suddenly be struck dead while playing - heart attack, meteor strike, whatever - and the banjo is not on a strap, there's a chance it will fall away from you. If it's on a strap, it will remain with you when you go down.

And the Church has ordained that a person found dead with a banjo on their person may not be buried in consecrated ground.

Peter.