I do both. Ironing the seams open reduces bulk, particularly when your sewing blocks together. However, if I press the seam allowance towards the dark, the seams "nest" better and it can be easier to make the points match. That being said, you have to be very aware of how those seams meet so they don't get too bulky. Some people make ironing charts. I have recently embraced ironing my seams open because they turn out true to size after ironing. I think when I press to one side, I am actually stretching the fabric just a little and the blocks come out a little short.
At the end of the day, do what you think works best for you.