In short, do you trust the temp it's showing given that it's not measuring the beer temperature directly? I've heard there can be a difference of a few degrees between what Fermometer is showing and the actual temp.
I completely agree with Yooper.It's "close enough" for me to be happy. I've found that is usually within a degree or so when I've tested it so I stopped testing it. I mean, if the fermometer says "66", even if it's wrong by two degrees either way that's fine.