It really depends on the sound you want. IMHO most tube preamps tend to be a bit more articulate and focused at lower gain settings. As you push the gain, the sound tends to lose a bit of detail and then transition through compression and light OD into distortion. So if you want more of a clean, focused and punchy sound, turn up the master and turn down the pre.
Whenever you push tubes hard it will cause them to wear more. The volume of the amp is dependent on the position of both the Gain and Master. If you crank both so the amp is distorting, then you will experience increased tube wear. If you crank the Master but run the Gain at a lower level so there is no distortion and the volume remains constant, the wear on the PI, Drivers, and Output tubes will remain constant. You may shift some of the wear and tear around in the preamp a bit because your decreasing gain in the middle of the preamp and making it up at the very end. The difference should be minimal unless your pushing into distortion.
Since your using OD and Distortion pedals, I would think you might prefer pushing the front end up the amp till it starts to grind a bit, and then running the Gain of the preamp up until you get even more distortion. IMHO running multiple gain stages on the verge of distortion often produce superior results to running one really high gain distortion stage. The distortion is typically more complex and less fizzy IMHO. The only way you can get the front end of the amp to distort is to hit it with a really hot signal, because the Gain control comes later in the preamp.