420 tire pressure?

I have rimguard in the rear tires on my 420. The guy who installed it said not to check the pressure and not worry unless fluid is leaking. One tire looks softer than the other.I have wheel weights, chains and put suitcase weights on the rear if I'm not using the wood hauling 3pt box I built. I don't mow with it. Should I check them? What pressure should I run? The manual says 5-10#. Where can I get a gauge that reads down to 5#? Well the 455 deal fell through.
Thanks,Rich