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.