I believe they should wear collars w/ tags plus microchipped! My Sweet Pea can wear a collar fine, My Tommy on the other hand always manages to get his collar into his mouth!
Very scary! So he only wears his collar when we go outside together, to the beach, etc. Just in case he slips away! Plus make sure that if you do have tags that you keep the numbers up to date. My dogs have their vets number and our cell number!