Jack - I (and "jdrocks") run 36 psiF and 42 psiR ALL THE TIME, on or off dirt. IF you keep lowering/raising tire pressures you are taking a chance on BENDING/ BREAKING a rim, or FORGETTING to reset it.
To repeat - I have NEVER run lower pressures than 36/42, since I got that 'pressures-advice' from the mechanic at the shop where I bought BIG RED in Sept '08, 105,xxx miles ago.
Good luck.