if the pattern recommend pressing open that is to avoid the bulk where points come together, if you choose to ignore this and press to one side you will have problems with it coming together correctly. there are always exceptions to every rule and sometimes in quilting we have to press them open. if you are really worried about what little bit of batt may some day migrate you could shorten your stitch length giving a (tighter) seam...but i do not believe pressing open causes this to happen any more than what migrates through the stitching holes caused from quilting. in the 200+ quilts i have made i've never had batting migration be a problem. if you use quality fabrics and quality batting it is going to hold up for generations