Well, as for "fix-up novels", I do my best to avoid those. However, that can't be completely done. Case in point, I've got one series I'm working on that'll need a capstone novelette to answer some questions left out in the other books because the series didn't go quite as expected (ie, it ended sooner then originally planned) so I'm having to write a short novel to close out the remaining storylines that weren't answered at the end of the final book in the series. Of course, I may decide not to do that novelette simply because it leaves the universe wide open for a few other series. Hopefully I'll be able to decide soon.