If made something I cared about and someone else helped improve it in a way I care about (if it's a giant project and someone corrected a tiny typo, I won't send them a handwritten letter about how much I appreciate them as a human being, it has to be genuine or it's patronizing IMO), I probably would thank them, partially in the hopes of encouraging their future contributions, and not just to my projects, but open source in general.
It's a simple and easy thing to do, if there's a text field right there, etc. and every now and then, when you give someone recognition for a small contribution, they might decide to make that big contribution they had in the back of their head for a while. The worst that can happen is nothing, you just paid your dues of basic courtesy. Again, that's referring to a contribution where someone actually took some meaningful chunk out of their day to help out. If they spent 4 hours doing it, I can spend 40 seconds thanking them briefly, which is still infinitely more than just nothing at all.