This sort of thing is why I think public chargers should be installed in inconvenient locations. Put them at the back of the garage, not right up front. People who need the charge will gladly take it. People who don't need the charge but want the free electrons can decide which compromise they prefer. People who don't care about charging and just want a good spot can take up better spaces.
I sometimes plug in when I don't need a charge, but I'd never use a spot without charging. There's a local shopping area a couple of miles down the road from me where I go pretty often, and they have free chargers. I plug in even though I don't need it, because they offer it, and why not? Plus, I figure that using it sends the message that it's appreciated, and helps get more chargers installed. One exception to parking without charging: there's one charger there which has been broken for ages, and I will park in that spot if the others aren't open.
I always leave a note on my dashboard when I do this. I printed out a piece of paper with two messages on it. One says "I am opportunity charging. I will move if you need it." The other says "I am charging to get home. I can move if you really need it." I put the appropriate side up when I charge, and otherwise it lives in my glove compartment. It includes my phone number on both so people can reach me. I've never had anyone call me, but knowing it's there makes me feel better, and you never know when someone might end up taking advantage of the offer.