I press open open only when I have a lot of pieces coming together at a point. I did just finish reading Judi Madsen's new book Quilting Wide Open Spaces and she emphasizes that if you are stitching in the ditch to not have your seams pressed open - she almost always SID. I know this is another one of those personal issues (do you prewash, use steam, etc.) but I really do think that if you are going to SID that you really don't want your seams open. Of course you could always stitch one or two threads to either side of the ditch and be ok.
BTW if you haven't seen her book Quilting Wide Open Spaces it is amazing and there are actually 4 cute patterns in the book. I am not associated with her but am a HUGE FAN and WANTTA BE