I press them open unless I need a ditch there. There are really only 2 reasons for pressing to one side:
-to make a ditch to quilt in
-to spin certain types of seams
I know there are some who claim that pressing all to one side "makes the seam stronger" but I've never had an issue with my seams coming undone unless I've used too long of a stitch to secure them. I made plenty of kids' quilts with all the seams pressed open (the method Leah Day teaches) and have yet to have any issues with my seams. But whatever works for you is what you should do. I don't think it will make a huge difference one way or the other.