I've done both and employ whichever method works best with the project. I might choose to open seams open if there is a large amount of bulk (ie a lone star where many seams meet in the center), or when using very light and very dark fabrics (if you want seams to nest sometimes you have to iron to the light side and if the light side is very light you get shadows), or the piece is very small (bargello for example often has 1/4" finished strips).
Other times I do it just because
