Short form: I get the best results pressing open and pinning. Your mileage (and preferences) may differ.
Long form: I do not find pinning to be all that time consuming. Yes, there is definitely time spent pinning and unpinning both. I can see where people could have accuracy problems without pinning. Watching people who use stilettos, that seems to take as much or more time as my pinning. Once pinned I can zoom along at high speed. I always pin the leading edge as well as about every 2 inches because I do get better results that way. I didn't always pin, but started so I could have something to do at my quilting group and I liked the results. Plus, I often drop things (or my cat would jump up on things) and pinning keeps them together.
I have never had a problem quilting in the ditch on my open seams and have done a number of quilts over the last couple of decades doing just that. One reason is that I do piece with a pretty tiny stitch and needle, (more than 12 stitches per inch, prefer a size 10 needle) and I quilt with a larger stitch, plus I am using a new needle (typically 14 or above). I would no more expect my seams to be broken than I would to punch holes through woven fabric. Quilting in the ditch was typically with my standard sewing machine.
I have pictures of heavily used quilts that are now in the dog crate. I had severe allergies for many years and had to wash my quilts at least monthly, as well as change the sheets and pillow cases at least weekly. I've posted them before and will try to find them again. You can see that my open seams and in the ditch quilting have held up well. What didn't hold up well is I didn't do enough quilting, oh the batting held up and in place but what wore out was the center of the pieces.
Nor did have any problems with my seams when using a long arm. When I was using my friend's long arm, I'd typically do one quilt for me and one for her. With her quilts yes, I had all sorts of problems because she was using 10 or less stitches per inch. All her outer edges (all of them, blocks, columns, rows) would have issues because of her stitch length. I will also run a stay stitch around the outer edges of my quilts if there are a lot of seams. Don't bother with single pieces of border. Typically the stay stitch is slightly larger than my quilting stitch, so maybe 10ish per inch. I run it within the 1/4" seam allowance, using my quilters foot with one fork off the fabric.