Seriously? I don't know about other SoCal stores, but the Manhattan Beach one is busy enough on a normal day that I'd avoid it if I could. When I went to pick up my watch I waited like 15 minutes for them to look for a bag, only to find out they had run out of them lol. And back in the days of the original iPhone launch, you could barely find parking let alone get into the store. They try to keep other transactions in a separate queue from the iPhone, but even then it will likely be chaotic.
Unless you love huge crowds, lines and/or the sheer spectacle, I'd avoid the store altogether for a day or two, or at least wait til evening when they've run out of phones and it's calmed down.