Right now I'm having a similar problem with a D4P, and whenever I can, I switch one of the seams back the other way so it will nest. It's annoying to have to do this, but usually a little pressing stick is all that's needed. So far I haven't found any on a D4P that couldn't be done this way. On the D9P there may be a difference. If you can work out where the problem is going to occur, maybe you can decide to just press just those seams open.
I have better luck with nesting seams because I avoid pinning whenever I can - just me. Whether to press open or not also depends on what you're planning to do with the quilt. If it's going to get a lot of use and washing, open seams are not as strong because there's nothing between the two pieces except the thread.
However, Bonnie Hunter, who makes lots of strip pieced quilts, says on Quiltville somewhere that she presses hers open. That's enough of a reason to do it that way, as far as I'm concerned. I love her work!