I guess it depends on what you're looking for from a particular story and how important canonicity is to you - I recall reading a shedload of fic from The Sentinel long before I'd ever seen any episodes and frankly, I'm glad it worked that way around. The show itself is so cheesy and the actors so unlike the mental picture I'd been given by the fic that I probably wouldn't have bothered if I'd ever seen the show first, and there are some excellent spinners of stories in that fandom.

However, I'd say that show was an exception for me - for the same reason I don't usually like wild AU's where they've taken characters and put them way out in another universe that doesn't resemble 'ours' (from the source material) in the slightest, it's important for me that the characters bear some resemblance to how I see them and the canon events of the source. Or that any change from canon has a logical reason behind it, if the writer's imagination is good enough to assist me in suspending my disbelief.

Horses for courses, though. After all, there seems to be an audience for pretty much anything! ;)