After we witnessed the depths to which "canon" Star Trek has fallen recently, a friend of mine suggested this as the healthiest way to think about the subject:

Everything made after Gene Roddenberry's death should be considered non-canon, fan-produced material. That includes the episodes of TNG he had nothing to do with.

Simple, neh? While this excludes a handfull of characters and concepts that are generally considered to be quality Trek, it also excises from the "official" universe a lot of horse shit, continuity errors and various other serious faux pas created by writers and producers who have made "getting paid" job number one.

But hey, if you enjoy "EXXXTREME Captain Picard with real dune buggy driving action" by all means disregard this.

If only there weren't grey areas. Conceptually DS9 was approved by Roddenberry. There are also plenty of episodes of various ST series' that have been "based on" "inspired by" or otherwise influenced significantly enough by Gene's writings that the lawyers required a special credit. Personally I'd cast it all into the wasteland non-canon material is left to die in if the act would somehow magically erase the memory of the last two TNG films from my mind entirely, but of course your mileage my vary.