Plumbing pipes should be supported every four foot…in regards to whether it should be supported on each side of the joint would be based upon the weight coming down at that joint. Metal straps are not that expensive…nor are 2x4 blocks.
PS. PT lumber should be used any time that a wood member can come in contact with moisture although if one reads the code carefully, it is possible to fore go PT lumber on still plates depending on the height of the sill plate in relation to the ground.
One of the biggest problems you find with PT lumber are improper fasteners being utilized…most framers (and DIY’s) fail to switch to non-corrosive fasteners when fastening other materials to PT. I have seen many HI’s miss such things…