i was a little skeptic of that spot myself, but it seems to be fine so far. It's being grounded through the bolt which goes down pretty far. I just might change it to the seatbelt bolt so i can sand down the paint. I couldn't fit it under the spot it's at now, so i had to place it on top and bolt it down.
I hoping it's not too much resistance, since it's only one amp on that spot. The wire is also ~2awg just for that amp. it says 1/0awg but, it just looks like they used 2 4awg wires together. I'm also surprised the amp's input accepted it w/o any mods to the wire.