I found running around (1440/720)x720p squeezed to 16.9(AR) to be the sweet spot for my BD808s.
Lower refresh rates also helped the picture quality, but with slightly more flicker.
I ran 50hz/59.94 from the scaler, 72(xxx)hz/75 from the HTPC.
I tried higher resolutions, but found absolutely no reason to do so.
I understand that multiples of the original source are easier for scaling, but thought "tuning" to your scan lines to be more important.
At around 720p(and 16.9 AR) i could clearly see scan lines from up close, but not when seated.
Since the Pj wasn't stressed in anyway(at that resolution/ratio), i found the sharpness/clarity/colours and overall image to be better.
I also ran a 4.3 AR, and found only then did the higher resolutions come into play.
Lower refresh rates can also make a big difference, with flicker seeming to be the only downside.(which you do get used too)
My BD808s had very high hours, but the tubes were in good condition.(slightest of wear to green only)
If i had put new tubes in, i would probably found completely different results.
Showing that there is no ideal resolution and refresh rate for any CRT.
"Suck it and see" comes to mind!