Adding a line

I know there are many posts about upgrade swaps and adding a line and what not, but i haven't seen an answer to my particular question (apologize if its here somewhere, i did try search)...

I'm one of the ones who's wife is eligble due to primary line and i'm not, so I'm adding the line, getting the phone, will swap it, and keep the $10/mo as a spare line with a cheapo phone. My question is, should I go to a store today or tomorrow to add the line somehow w/out a contract (if possible) as I won't buy a phone, or just wait until I get the new one either at AT&T, Best Buy or Radio shack and have them do it there?

Thanks