For game issued or replica jerseys, get them signed where you think looks best. Some like to have jerseys signed on a number. Some like to have them signed on the front down by the tags.

For game worn jerseys, my strong preference is for no signature at all. To me, a game worn items looks best in the state in which it came off the field. The fact that it was worn by the player in league play is personalization enough for me, and a signature anywhere on the upper parts of the jersey is a negative. Signatures that are relatively small and near the tags are okay by me, but still not preferable to having no signature. Another option for someone who wants to have a jersey signed but not evident is to get the player to sign inside the jersey against the back of a patch. This is ideal for signing hockey jerseys that have large crests on the front.

Of course, it's your jersey, so you can do with it what you want. However, if you intend to sell or trade a game worn jersey in the future, you run the risk of turning off potential buyers with a signature. (Though some may in fact prefer it, I think it's more of a gamble to get it signed.) Again, if you really want to get a gamer signed, avoid any personalizations and have the signature placed down near the tag.