Well, you certainly can gauge there if you prefer. I temp at the lid only. Could not care less what the grate temps are. Position a probe too close to the edge and temps will read higher from the airflow there; too close to cold meat, lower. I can't be bothered.
If you want to go with grate go with the upper (if using both), position your probe as best you can, and don't bother temping the lower. It simply doesn't matter.
I suggest picking one place - lid or upper grate - and temping there only. I much prefer the lid because it measures exhaust. More than good enough. However, again, pick one or the other and then get used to working off of it. Too many get caught up in the notion of the importance of cooktemp accuracy (it isn't) and disparities of temp at two or more locations (it does not matter one whit). To me, this can (and does, judging by some posts here) diminish the enjoyment of cooking on the cooker. Picking one spot removes the issue of temp disparities - since you only know one temp no disparities are noted; good. Getting used to one spot and working off of it lessens frustration. I like the lid - in a vent hole; I do not like installed therms - because there are no proximity issues like there are on the grate.