I try hard to open all my seams, also, however the steam iron is getting to me. I still have to use my big old iron, I haven't purchased one of the smaller head irons, yet and bnot sure when I will. It's hard to buy all the fabric I want. But I digress,
There are times, even when I pin the seams open and join them that one edge or the other will jump in the wrong direction and I sure don't take out all the seam when it's a whole block. I just steam it more and longer and keep on goin' I think it makes the joints (seams) stronger if I go ot one side or other, but in planning, I normally plan open for a flat look.
Just mho.
Sharon