I blew my upper radiator hose mount off the radiator last month and drove back through the northern california moutnaitns with the cap inthe first indent and never even got warm. in fact, I'd swear it ran cooler than it di before but anyway. No problems at all.
I think I'd pop the cap to the first indent and watch the guage. It's gonna have to get pretty hot to cause you to have to tighten it down.
Unless you get the pedal to the metal and stay that way for a while like I did... I still say I was blowing away that Lexuas with my old non-turbo 300 when she blew!