If there is going to be a lot of bulk I definitely press my seams open. And I've been pressing them open a lot lately even if there isn't a lot of bulk. I think it helps seams match better and if you're sewing by machine the seams aren't going to pull apart. If you're making stars or something that has lots of points coming together, definitely press them open, it helps the whole thing lay flatter and it makes a much prettier point.