If I'm pressing a seam open, I use 1.6mm stitch length. If I'm pressing it to one side, I use 2mm stitch length.
I definitely agree with all those who've suggested stitching in the ditch & quilting closer together -- if it's going to be a bed quilt, I wouldn't recommend quilting farther than 2" apart.
The other thing to consider is how you're laundering it. A queen-sized quilt will get damaged in a top-loading home washing machine. The center agitator will pull on it too much, snapping stitches at any weak points. I take mine to the laundromat to wash in a commercial front-loading machine. In between washes, I vacuum it with a piece of hosiery over the end of the hose. Also, make sure to never use fabric softener -- especially liquid fabric softener -- as it will damage cotton (including cotton thread).
Finally, I personally love, love, love 100% cotton quilts, but quality polyester thread might help your seams. It is less prone to breaking than cotton. Otherwise, be sure to use a good cotton thread. My personal favorite is Aurifil 50/2, but Gutermann 50wt thread is also very good & is available at JAF.