I'm really new at rootstock grafting but I think it depends on what you want to do with it. If you plan to graft directly on it soon and you know for sure what size your scion is then I'd suggest picking whichever branch or main stem seems closest to the size of your scion and remove others. Considering your zone I'd think the sooner the better. My books say best to graft at fullbud. But of course I repeat I'm a newbie who read lots and lots but is sadly lacking in actual experience.

Pam in cinti

Thanks for the advice. Do you think I need to wait for the rootstock to get thicker (it's about half the diameter of a pencil right now)? Or do you think as long as I have a scion that matches the size it will be fine?

I think you need someone with more experience to answer. My sources strongly suggested practicing on extra stuff first. So I spent several hours working on prunings. In my opinion stock just slightly smaller than pencil is easist to work with. However my book showed grafts done on mere seedlings. I bet that takes a fine steady hand. If you have a great deal of stuff to graft onto those smaller stocks then give it a try. If, however, your scion stock is limited then you'd be better off both practicing and letting your rootstock size up a bit.

For what it's worth from a newbie.

Pam in cinti