Man, have I screwed some bolts.
I personally don't see a bunch of difference in bottom or top as to whether it might back off. With the bolt on the top what keeps the nut from using gravity to work its way out. I don't think gravity has anything to do with it.
I do think the engineering difference if there is any is that having the nut away from the possibly flexing part is the more secure way In this case I doubt that the idler arm really flexes on the bolts if its all properly tight.
Bosch Master, ASE Master, L1
33 years MB technician