Well, following physics, heat rises, even in the confines of a small space. That being said, when I'm doing something on various racks, I always put my meat probe in whatever is on the lowest rack.
Does it make a difference? I don't know but I'd rather over cook what is on the top rack just a bit rather than under cook what is on the bottom rack.
Of course, I am talking about doing the same meats on the various racks. I do understand if you are doing different meats that require different IT's. If that is the case I'd highly suggest investing in however many meat probes you'd need.