I don't know that there are any general rules about photographing quilts, but you should ALWAYS ask before taking photos. Whether it's at a small show, or a large one, whether it's show quilts or vendor samples, you should always ask before taking pictures! And if you plan to post them in a blog, ask for permission to do that too!
At larger shows (like Houston) you may be allowed to photograph some, but there may also be some special displays with signs that say "No Photos". Respect that!
Have a great time at the show!