Idling mid-to-high 30s is fine, mine's about 35C give or take a couple of degrees at idle and yeah lowering the voltage won't help that much but it will produce a tad less heat. I'd say at full load, 75C or over is too high, and at idle, with a cooler like your's, anything above the 40/45C mark would start to worry me. On the stock coolers though, 45C at idle is pretty normal.