Always inside.
“Around the neck and next to the skin” is the direction for Canadian military dog tags, and that’s what I’ve always done. (I’m not Canadian, but served with a lot of Canadians.) So if I’m wearing a t-shirt under my shirt, my medal(s) and/or crucifix (I just got a new crucifix/St. Michael combination from a friend) are under my t-shirt. If they fall out when I’m exercising, mowing the lawn or whatever, I’m not in a big rush to tuck it back in.
(The only time I haven’t worn my chain with St. Michael/St.Christopher medals I got from my grandmother was when I went away to bootcamp when I was 17. Rules say it would have been allowed, but I didn’t want to misplace it.)
In bootcamp, I got used to the feel of dog tags around my neck, so when I “graduated”, I just added the chain with my two saints. I never take my religious symbols off, not when sleeping or showering, never.
My general doctor (not Catholic) is used to them. He moves them out of the way if he needs to, but he’s never asked me to take them off.
(I wear my dog tags (also worn under my shirt) now only when I’m traveling far from home.)
Arthur