Sometimes movies are cutting off the picture if they display in widescreen. House on Haunted Hill (1959) for example. Sort of the Back to the Future movies (they actually add picture for full frame). Terminator 3 is an oddball where you don't get the full picture with either format, but full frame gives you the most.
Typically, I am for whatever format gives me the most picture information.
I find it difficult to believe Seinfeld can be converted to proper widescreen, but Star Trek cannot. My cynical side suggests its all about money (either not wanting to spend it or waiting to double-dip for it).
Regarding Seinfeld I think it looks terribly warped in widescreen. At least the last time I attempted to watch the mangled versions on the superstation.
I generally prefer the intended aspect ratio for most shows though some DVDs have lost some of their gratuitous nudity compared to their VHS counterparts.