Believe it or not, there is a difference between ironing and pressing. Ironing is a gliding movement back and forth with the iron. We do this to remove wrinkles from blouses, shirts and pants. Pressing is an up and down movement which we use to set in creases on pant legs. Pressing greatly reduces the possibility of distorting the fibers and stretching the fabric.
When I first started quilting (back in the Dark Ages it seems), I was taught to "press" my seams flat with the right sides together to set the stitches and then, with the darker fabric on top, flip the dark patch upwards and gently butt the tip of the iron along the seam line on the right side while pressing the seam to one side. For many years I followed this rule using a dry iron to further prevent the possibility of stretching the fabric. Then I encountered a teacher who advocated the use of steam, so I tried that for a few years.
I am now back to using a dry iron unless I am trying to wrestle a piece of fabric into submission (better known as shrinking or perhaps stretching a piece of cotton to fit the next patch or block). I truly believe it makes a more precise piece of patchwork. (Notice that I did not say a "perfect" piece of patchwork since mine has never been accused of that distinction.)
I never questioned the practice of pressing quilt seams to one side until recently. I came to quilting from garment construction and assumed that we press 1/4 inch seams to one side because they are hard to press open unlike 5/8 inch seams used in clothing. I have learned that the reason for the pressing to one side developed when patchwork was done by hand. Hand stitches are not as even and as close together as machine stitches. Pressing the seam open would allow the filler (batting) to escape through the gaps between stitches. This is not a problem with machine stitches which are typically 12 stitches to the inch.
I am currently piecing my second quilt with seams pressed open and liking the results. Yes, it is a bit trickier to press them - I use a tool to "finger" press them open as I go. Seams lay flat and matching seams is a lot easier for me. (I'll talk about matching seams in another post.) One caveat: if you plan to do stitching in the ditch to quilt your patchwork, DO NOT press the seams open.
Here's an I Spy quilt that I made for my grandchildren and a detail of it.