Depends on the track conditions, and the wheel design that you are using. With beadlocks, I run 4-6 in the rear, depending the traction conditions, and 5-6 in the front. I have tried other pressures, from a lot more to a lot less, but this works best for me with my tire and wheel combinations. I do a lot of SX and AX races though, and the tracks can be very tacky, I need the extra pressure to spin the tires when I need to. I may drop to 3 in the rear on a MX track if it is really loose, to help hook up. Just my opinion.
If you dont have beadlocks you MUST run higher than 6psi. I ran 7 all around until I debeaded a few tires so I tried 8 on for size and marked my tire and wheel and the wheel was still spinning inside the tire so I moved on up to 10 and its just fine.
I ran 6psi in the rear with omf beadlock 9inch with 18 inch tires and bent the shhhh out of my inner lip. I didn't come up short on any jumps and only landed flat once, but wheel was bent in more than one place. Im taking them to omf to get trued and re-inforcing rings installed. Gonna run 7-9 psi now lol. Might of been because with the 9 inch wheel you dont have as much tire to soak up.
I have mxr6's as well. Mark your rear tires and wheels with a solid line then ride it and you will see why I'm running twice as much psi as you. Under rapid acceleration and through the corners the wheel will spin inside the tire. And Husker is right track conditions are a factor
My son will run 10-12 in the front and 8-10 in the rear. Too little air pressure will make the quad stick on impact( bent wheels, too much tire roll, ect.) and to much will make it bouce on impact or slide in the corners with no forward bite. There has to be happy meduim between shock set-up, type of tire( ply rating),type of track (dirt) and air pressure. I had to learn all this at some of the pro riding schools my son was invited to after he started racing the Nationals. From our time racing I have come to realize how critical all this is. Corner speed is a must!! The right set-up can give you really fast lap times or bring you to a crawl. You should keep notes after every moto of every race. Just my .02