If you have cool season grasses, (fescue, bluegrass, etc.) fall is good, spring may be better. If you overseed, it's good. However, if you have warm season grasses (centipede, bermuda, zoysia, St Augustine) I honestly think spring is the best time and would not do it in the fall. I am a row crop agronomist by trade, not necessarily a turf specialist, but feel that aerating before cold weather conceivably could make the root systems of the warm season grasses more vulnerable to cold and freeze damage, my $0.02.