I would rather hunt in knee high grass than ankle high grass. I does not take much elevation to see more than just their head and neck, especially if you are sitting on a stool. In a few weeks we will have knee high wheat fields, I have spots that I don't call until the wheat gets that high. The first coyote i killed today was in knee high grass, he came in bouncing, trying to see the sound source. They also have more confidence coming in, just like waiting for the wheat fields to get knee high and all the vegetation can help slow them from hard charging in and out with poor shot opportunity. I always look for some type of "block" to set the call in or behind when I pick a setup. Sometimes it pays off, other times they kamikaze right into the brush pile or thicket, in thick grass I don't worry about it as much.
I'm in Oklahoma too and a lot of my areas are very flat. If I can't find elevation I look for a depressions like creek beds or find edges of different cover like tree rows or agricultural fields. Sometimes you just gotta say screw it and pass on the stand, especially the ones where if you called one in you would not even be able to see it...