If you are in an area where the onset of heavy leaves and a potential for snow accumulation is on the seasons agenda you'll want to mow low to about 2.5-2.75 inches.
Mowing low for fall will reduce the impact of snow mold and or matting. A nice tight mowing will also help to keep the leaves from embedding themselves into your lawn and will likely blow right over. Your neighbors may not appreciate this effort but could save you some raking...
Mowing low is a process and should never be accomplished in one cutting. Take the height of your grass down in increments. 3.5", 3", 2.75" over the last few weeks.
Avoid scalping and removing more than 1/3rd of the blade in any one mowing.