If you put them near the cooking grate you have a few issues other than the one Bruce mentioned. If it is over the grate, then it will get in the way when you want to remove the grate. If below the grate, it will get in the way of installing the second grate plus it will get fat dripping on it which I think will distort the temp reading.
Believe me, BBQ is not that sensitive to a few degrees difference in temp. In the lid is good. If it's a few degrees different than the grate, no big deal.