The knock on JR has always been his inconsistency and jacking up too many shots. In the starting 5, he would be kept in check in this regard - no way Melo and Amare would let him take shots away from them. Yet, he would definitely get a lot of open outside shots playing with those guys, and perhaps most importantly, he would SPREAD the floor so Amare and Melo would have more room to work downlow. This is why I think he would be so great in the starting lineup.
As far as when Shump comes back, I love Shump's upside and don't have a problem with him being in the lineup, but only if he can bring his outside shooting numbers up. He seemed to really be improving his 3pt numbers at the end of last season, so I think this is a possibility.
Without JR coming off the bench, I would be a little concerned about who would take the shots with the shotclock winding down. I think it's important that there's at least one guy who can create his own shot. Who would this be - Priogione? Brewer? Novak is awesome from the perimeter, but he needs guys to set him up. If defenses key in on him like Miami did in the playoffs last year, he becomes ineffective unless someone else steps up. Also, there was some good chemistry between JR and Novak last year, so I think there is something to be said for him coming off the bench.
Overall, my ideal would be JR starting until Shump comes back, with Shump elevating his outside shooting to spread the floor at that point. And JR improving his shot selection. Overall, I love JR's energy (especially on the defensive end), and I think he was a steal at the price we got him.