I press most of my seams open due to that wonkiness you sometimes get if they are pressed to one side and then stitched together...it takes me as much time to press them open as it does to figure out which way to press each seam so that they work together instead of against each other!
However, that being said, IF and only IF I know for a fact that it isn't going to but up against another seam...I will save some time by pressing to one side!
HST's (half square triangles) I almost ALWAYs press open... it just saves a headache later on....they take alot of time but to me its worth it in the long run!