I'd always suggest looking at how the original guitars have them placed. The factory position on an Explorer is normally in the position you mentioned, but you will see pictures of them with the strap button on the front of the top shoulder. The downside of the shoulder position is the slope of the shoulder means that with a standard strap button and strap, the button is being pulled forward out of the strap slot by gravity, and with a soft strap end, you're likely to have the guitar drop off the strap at some point. So if you do fit one there, I'd really recommend that you use some form of strap lock. I always fit strap locks to my guitars, but if you don't, then the back of the neck is a safer position, even if it can intrude slightly on upper fret access.