I actually canceled a sizable ($1000) contribution early in a campaign. I got excited, pledged, and then talked to a buddy who actually has subject area knowledge in what the campaign was trying to do. He raised a bunch of good questions and I never got any good answers back from the campaign, so I didn't feel comfortable it wasn't vaporware and I withdrew the pledge. In retrospect, they were focused on the gee-whiz signed photo and coffee mug level prizes whereas I was interested in the actual end result hardware for science, and I think whomever was running the campaign just didn't communicate well.
I would feel very awkward waiting until the last minute and certainly if they were on the edge of the goal, but in that campaign's case I canceled in the first two weeks and they made their stretch goal anyway.
I'm a hardware engineer so I'm just incredibly suspicious when really cool hardware pitches can't answer follow-up questions with sufficient details. Lesson learned: I'm an impulse buyer so I'm not allowed to pledge money on Kickstarter without waiting twenty-four hours and talking to a subject matter expert.