With the release of MT 3.0 not too far off in the distance, I decided that I probably should be running the most current 2.6x version of MT. The two most serious objections I had to using MT 2.661 were that 1) the MT-Blacklist plugin did not yet at the time work with MT 2.661, and 2) comment author links were changed so that they redirected through MT.
MT-Blacklist v1.63 is compatible with all MT 2.6x versions, so I upgraded from the incompatible 1.62 version.
To address the comment author link redirects, I installed the Optional Redirect plugin, which reverts MT 2.661 comment author link behavior to what it was in MT 2.64 (no redirects).
With those objections no longer a problem, I re-upgraded the blog to MT 2.661 this afternoon. So far, it's been working fine and I have no complaints.