Yes, a plaster is always a good idea. The glue on the Libre is sticky enough to keep it on for the full two weeks (people had problems in that regard when it was first introduced but they seem to have changed the glue now), but popping a plaster on top smoothes out the edges so it's less likely to snag. There's lots of option, tegaderm and so on, but my local chemist does Molnlycke Mepore 7 x 8 cm at 17p each and they do fine.
I lost my second one to a doorjamb (after three days..aargh!) and realised that I had put it too much to the side of my arm where it was more exposed, so since then have made sure it's far round on the back of my arm so it's less likely to catch. People at work think I've got some sort of phobia about doors!
You can also funk it up a bit. There's a small cottage industry doing fancy plasters at a price:
https://www.zuckerschmuck.com/english/fixtape-to-paste-over-libre-enlite-catheters/
http://shop.pepmeup.org/category/freestyle-libre-sticker
https://www.funkypumpers.com/product/freestyle-libre-sticker/