I recently put a ! in front of a folder name, for the sole purpose of putting it at at the top of the MAT folders in one of my products. It's located in MaterialsBionicRoosterProductName, so I saw no harm in doing it. I would never do that in the main library, only within the confines of my own folders, and only in certain circumstances. I've never done it before, but saw the need this time to keep a Bonus MATs folder at the top of the list instead of mixed in with the rest, somewhat hidden.

i don't see any harm in that.