Skyjob, I think that Derfred has spent a great amount of time to comprehensively explain how and why most of us configure the pressurisation panel as we do.
I want to ask you a question though based on what you said before, as I just can't get my head around your logic.
Pre departure, we set FL390 on the pressurisation panel, as at some point 3 hours away its in our flight plan. Initial cruise is FL350 and we set this in the FMC. During the flight we step to FL370 and decide that the wind is good, 390 might be a bit bumpy, etc, maybe there's traffic resulting in 390 with a speed restriction, whatever, so we elect to remain at FL370 until TOD.
So we have 2 options;
1) We forget, as I pointed out before, and trigger an off scheduled descent master caution...Thereafter play with the flight altitude setting (action the QRH) and totally confuse the pressurisation controller.
2) We reset the pressurisation system to FL370, and therefore trigger a climb in cabin altitude as the differential pressure reduces, and this seems to be bad to have in your opinion.
Oh and that nicely leads on to the fact that you were cruising at FL370 for 3 hours with a differential pressure higher than what you should have had for flight at FL370...