I would always go for mono pre-1968 and stereo 1968 and beyond. By 1968 stereo mixes were given more attention and regarded as more than an afterthought. Earlier stereo mixes were often ham-fisted, with instruments/vocals separated into channels and the 'novelty' element of stereo emphasised. There are several '67/'68 UK psych LPs where the stereo and mono mixes are significantly different, but by 1969, mono mixes tended to just be 'fold-downs' of the stereo mix.
Notably different mono/stereo mixes feature of Blossom Toes first, Piper, Pepper, Tomorrow, SF Sorrow, Family Doll's House and probably others that escape me at the moment.
I agree PP though there are exceptions like the Tomorrow album from 68. I think the mono version of that sounds much better than the stereo. Though I guess that was reecorded in 67 and the stereo version was ballsed up anyway.
I have quite a few 45s on Fontana and Philips which were released as mono mixes as late as 1972. I wonder if the recordings themselves were several years older, I have long suspected it.