If it slips, feels bulky, or you can bite through it's a hinderance.
BTW I've seen a couple of players with some sort of white plastic slip that they put in their mouth over their bottom teeth, so I guess it sits between their teeth and their bottom lip. I don't need one, my bottom lip rarely suffers even after a 3 hour band practice, but what are they for??
Try reeds direct and ask them for the 0.4mm clear ones made by KLOBL (unsure of spelling).
These are the best I can find by a mile.