I think the lower the better! Maybe not for 100% of shots, but in many cases I think it
looks great. I think the background of a shot should be the "background" of the location,
not the ground behind the subject! Since you are shooting on a perfectly smooth
artificial surface you could try cropping off the bottom a bit more?
I usually shoot a handful of shots from this perspective at every event. Almost all of my
other shots are from my knee with the camera as low as possible. Usually when I look
back at the pictures I find that nearly every one could have looked better from a lower
perspective, IMO. The only real downside to being on the ground (to me) is lack of
mobility. On some of my triathlon shots I will actually kneel/lay with the camera at a
lower level than the ground the athletes are on! As long as the feet don't get cut off I
like the look.
Here are a couple of my shots from directly on the ground:
I left a bit more ground in the framing because it gives the subject "somewhere to go"
when they are moving directly at the camera. I think the shots you have posted so far
look
great from that perspective!