The problem with using anything that etches is that you're removing material from the unit. This can in turn affect the structural integrity in the long run. This is somewhat counter to why people prefer glass fermenters; they will resist scratching better than plastic. Etching is essentially controlled scratching.
A few years ago I took a bumper sticker and cut small strips and used them as gallon markers. I took my time and made sure there were no bubbles and got the corners flat. Its been like that for a few years without issue.