Loads of factors. I usually press to one side. But advocates of pressing open insist there are no issues and their quilt tops lay nice and flat.
But it presents an issue if you want to SITD as Dunster pointed out. There is no ditch with open seams so you will have to ditch a thread width over so you are actually quilting on fabric not a seam.
Judi Madsen of Green Fairy quilts and Margaret Solomen Gunn of Quilts of Love, both amazing, award winning longarmers have written blogs on why they don't like seams pressed open (mostly for the ditching aspect). But they deal with these quilts anyway.
It really boils down to personal preference and another factor to consider is bulk. For example when several seams meet in the center, like a pinwheel block, kaleidoscope block, LeMoyne Star, split Mariners compass to name a few, I will not stitch into my seam allowance in the center so I can swirl that part open. When joining these types of units, I will often start at the center and stitch out to the edges.