I do the same thing. If i'm replacing fluid, lets make sure i've done all I can to replace 100% of it.....probably impossible to really replace it all, but an honest effort to do so.
By the time the first flush is complete, the mess is already made, and I have all my painted surfaces covered with aluminum foil. I learned, the hard way, to cover way more of the painted surfaces than necessary. Aluminum foil is more durable than saran wrap, so I use that.
Same idea with the fresh fluid. Im sure its very clean , after the first pass, but why not spend an extra 5.00, in fluid, and be sure?