Historically, whenever I would digitize a mono LP using a stereo stylus, I would always do the transfer in mono. This is because when you sum a stereo signal, if everything is in phase, the center increases in volume by 6.02 dB relative to what is on the sides. The sound of the stereo stylus travel is out of phase, and it virtually disappears when you sum to mono. Also, left/right anomalies such as inner groove distortion or minor clicks are reduced in volume by 6.02 dBs.
But lately, I have been playing around with transferring the mono albums in stereo, and then using Audacity's "isolate center" feature. This has virtually the same effect as summing to mono, and may result in an even better signal-to-noise ratio.
I'm curious to know if anyone has tried this Audacity feature, and how it has worked for you.