By all means, unless you are 100% sure that they'll never get outside, either collar them or chip them.

My cats are allowed outdoors and I'm fanatical about making sure that they always wear collars. I use breakaway collars and the kitties do occasionally lose them, so I have backup collars ready the moment they come home naked. (of course, since you have indoor cats, you'll be able to find the collars)


Also, make sure you have some nice photos so that, worst case scenario here, if they do become lost, you are able to make posters with their pictures on it.