Your instincts are very good. I spray the backing then layer on the batting. Get these two layers smooth ( backing and batting ) before anding the top. Spraying the batting or the back of the top , can have mixed results.. this depends alot on the batting used. When in doubt spray the back of the top. If its a large quilt , I fold the top into 1/4's , and apply the top in 1/4's to the batting. Lay the first 1/4 get it smooth as possible, open the next fold smooth and then the final . Working with a large top that has been sprayed can be problematic if you try to position the top all at once.
Some find hanging the backing on a wall spraying and adding the layers to work for them... I find I have more issues that way with big quilts , maybe because I am short.