When I asked about this I was told 'it depends on the slaughterhouse as some have their own requirements'. Sure enough, the one I used asked for metal ear tags so I put them in when the gilts were still quite young. I had visions of how impossible it would be to hold a porker still to tag it so it seemed like a better plan at the time. In the end, because I tagged them whilst they were eating they didn't even flinch and just carried on troughing, lol.
Just my own opinion of course, but at least with metal ear tags you can be more sure of getting back your own pig. I have known people discover a mix-up after the pig comes back from the butchers in joints and then have the annoying problem of the butcher blaming the slaughterhouse and the slaughterhouse blame the butcher.