I change mine every so often to keep them fresh :-) It does help, I haven't been pinning much over the summer - but people still keep re-pinning & following my boards. So it still works for your even when you are not active :-)
Sometimes I see a perspn's Pinterest board & really love the cover - when it I click on it, it takes me to all of their images for that particular board. Sometimes people have thousands of images & I would really just like to see the one on the cover - wish Pinterest would make that pin first so it is easy to find!
If you are interested, here are my boards:
SG Pinterest Boards