I suggest that if you ARE going to get some, get the ones from invisible shield or whatever that cover the WHOLE front, not just the part that is the screen. Also, don't get any that require you to use water to put it on, they're nothing but bad news. Static>Water especially on electronics. For car stickers/window stickers, water works best, but for electronics that's a no no.
Just throwing this out there, I don't use one. I had no problems until one day in Kansas when I had to help out with a bad car wreck, then I wish I had one. Lady had to use my phone, and she had sand and glass embedded in her from heel to ear, quite literally. I mean, I didn't care, and still don't, but those are the 2 things that WILL scratch your screen with no problem.
Unless you're actively doing things that cause you to either break your phones or whatever, I say you're fine without a plastic film on it. Other wise, yes, get one.