.37 and .67 stick in my mind as good benchmarks. I ran .84 for a very long time, and I'm currently running .99... Has the Golden Tee games, Robotron speed emulation is corrected, and still has Hiscore.dat support that isn't a kludge.
I could never grasp the need to keep snagging the latest version, particularly when it seemed they were coming out once per month. Emulation keeps getting slower, old games get messed up, and new ones that are added either aren't playable or not even appealing.
I think the first build of MAME I ran was .37 and it ran like lightning on a Pentium 133. I suppose a good guideline would be to find out what build of MAME was current when your system was pretty cutting edge.