I haven't used a screen protector on my phones since the iPhone 3GS days. I didn't need it when I switched to the LG G3, and my S6 Edge was fine.
...Wait, it was fine until I used the S6 Edge with Samsung's S-View Clear Case. That bloody thing left marks all over the screen! It's well-documented what that case does to the S6 screen, and Samsung even owned up to it.
Now I have the S7 Edge (yay! an unscratched screen again!) and I'm using the new S-View Clear Case (because it looks awesome and I'm an ***** that apparently likes abuse). So far no scratching issues, but I've tried a few cheap screen covers from Amazon and they were all rubbish. I'm thinking, just to be on the safe side, I may try another simple cover until the Zagg one comes out. After that S6 Clear Case fiasco, I'm really hesitant to have the phone go commando.