I know a James who went by Jamie up until Kindergarten, when he suddenly decided his name was "James" at school, trying to be a grownup I suppose, so his friends called him that, and his family continued to call him Jamie up until he got into his teens. I know other James'es who went by James their entire life, even in childhood, and that is what their parents call them.
My husband wants to name our son James, and I think we are going with that, but the nickname issue has come up. I think I will honestly wait and look at the baby, and see what name or nickname seems to fit him. For example, sometimes you grow close to someone, and if their name is Mary, all of a suddden they just become "M" to you, in an endearing way, or "Emmy" for M. So I think you just never know what you are going to want to call someone, until you get to know them, but it is nice to think of your options first, to be sure they are there!
So for us, these are the options I'm thinking: J, which I like better than Jay....I may just stick with James.... maybe JB because the middle name will likely have B. Or I may go with a nickname that shortens his middle name. He can always go by something else later. I know at least two James'es who made the Jamie to something else transition (James, Jim, etc) later. Into adulthood, I do like names like JB or JR too though. I knew a JR growing up, and I always thought it was a very distinguished sounding name, with his last name, which had multiple syllables.
I'm not a big fan of Jimmy, but I am remaining open, maybe the kid will just look like a Jimmy. I did know a highschooler who went by Jim, and that seemed to work okay for him. Right now I think I'll avoid those two though, just not my style.