Well, there was that one time a racist bum I happened to be following reposted some cool art, so I had to retract that Like. But even in that case, I was able to learn about a new artist that I could track down and follow. See, something good out of the dumb shit.
Anyway, don't hold back on Likes, they don't cost you anything and make whoever spent the time making the post feel a little better. Besides, you also get a signal boost and you have positively added yourself to the Big Data algorithm in the World Wide Web. Clicking that Like means you are one step closer to a better world...or maybe you aren't. Maybe you shouldn't even be online that much. So click or don't click. It doesn't matter that much.