The seat belt bolts are rarely a good place to ground your equipment. Sometimes there will be a thick tar like substance (used for waterproofing) that will prevent you from getting a proper ground. Even if there is no sealant, the connection may be less than perfect because the hardened steel seat belt bolts (relatively high resistance) go into a hardened steel nut that's pressed into a piece of stamped steel that's spot welded to the bottom of the vehicle (not exactly a great electrical connection).
But hey its your choice dude, if it works it works i guess