For the most "pure" sound to the original recording's intent, straight stereo is best - after all, that's how the records were recorded. Mono recordings also sound best in the stereo mode, as the identical signal will come from both speakers.

In the old days, preamps had a "mono" mode which routed identical signals to both the left and right speakers by summing the left and right phono inputs. What this did was cancel out any vertical stylus signal from the cartridge - this signal was mostly rumble from the turntable. I've not seen a mono mode on home theater components.