If you are from the garment industry, we used to iron our seams flat, spread open, but for quilting, I have been trained... and this has been a question answered before, you press to the dark side of the seam. to not show the dark through lighter fabric.
They have the neatest ironing tools. Check out your local stores. little flat irons are sold now, look like what I used to patch linen drawings in the 1970's. Draftsman then. really nice, will not stretch fabric from over pressing.
So, my experience with lots of classes and last time round with this.. keep seam fabric together and pressed to darker fabric side.
good luck !
Ell