I'm literally on the fence with this. For matching seams, I like to press to one side and bump the edges together. For the rest, I like the flatter look of pressing open. If the block has more than 2 or 3 seams intersecting, I press open. For flannel or home dec fabric, I press open.
And guess what else I do! Sometimes I press to the side AND press open on the same quilt. I know, I know. I'm just wild and crazy like that!