Will this be the final mix for the movie? If so I think you would really want to get it right if there's any kind of budget involved. Having said this I doubt 5.1 is critical for the music - it's more for special effects and to simluate movement/direction.
If mixing in 5.1 is going to wind up way to expensive for you, you could also try mixing in stereo and using an upmixing plugin that doesn't mess with the image to spread it. Some upmixing plugins will ruin your mix for future downmixing by adding out-of-phase elements that screw with the LtRt mix, but I believe Penteo and Nugen Halo can be counted on to do it properly.
Having said that, flying blind (or in this case, deaf) is not your best choice, so even if you can't afford to go to a surround studio to mix everything, at least go for a couple of hours to print the upmixed version so you know what you're sending!
Just installed the two-week demo of Nx and I have to say, it's pretty amazing. My crappy webcam confuses the head-tracking sometimes, but generally I get a pretty good sense of where things are in the surround field, especially if I don't turn my head too quickly. I'll be buying this one.