Interestingly, I asked the same question to James at PRC in an earlier thread (before the crash), and never got a response back from him. I did get some comments from other karters with tritons and they didn't say which was preferred, but that they are being run both ways. If you don't get a better response this time, I'd try it both ways and clock the laps to see which is faster. I have two tritons, one with it loose and one with it tight. The loose one runs a tad more free, with the other one a bit tighter in and out of the turns. If you run it loose, be sure to safety wire it to the frame at each end (ex. at the bolts for the rear of the rear floor pan....make a clip for each side). It's just another way to tune the handling characteristics.
You're going to want to run it somewhere in between. You do not want it so tight you can not move it and you do not want it so loose it's jiggling loose. A nice, snug fit, with some play is perfect. Too tight may cause the chassis to bind and too loose may cause the chassis to over-flex. We test and run our chassis' with a snug rear bumper.
On a side note, please be sure to safety wire your bumper in case of it loosening during competition.