I usually just re-iron the row to which I'm attaching new pieces. That's worked okay for me so far. Also, make sure to use PLENTY of starch/sizing.
I'm not entirely sure what you're asking, but I'll share the method I just used for creating my modified 9-patch (9-patch plus 4 corners; so it ended up with a row made up of a 2" square - 6x2" rectangle - 2" square; then a row of a 6x2" rectangle on its side - 9 patch block - 6x2" rectangle on its side; and then another square-rectangle-square at the bottom.
1. Cut strips of fabric in needed size x WOF. In my case, that was 2.5" x WOF.
2. Strip sew sets of color-white-color and white-color-white (need twice as many of the former as the latter).
3. Set & press those seams (open, in your case).
4. Slice the pressed strip sets into the desired size (in my case, that was 2.5" to make a 2" finished square).
5. Sew together Rows 1 & 2. Set & press seams. Then add on 3. Set & press seam.
6. Sew together block 1 & 2, then add on 3.
I get the 2 gallon Ziplock bags to store my blocks flat if it's just going to be a day or 2 until I can get to it. Plastic isn't good for fabric long-term, but a couple days shouldn't harm it. I number the Ziplock's (the same way I number paper plates for applique work) so I can keep reusing them for each new project.
And finally, I really truly do love my Clover Mini Iron 2. It is so perfect for pressing seams. I set up a mini ironing surface right next to my machine so I don't need to keep stopping. And I don't press my seams until I'm ready to sew on the next piece so I can avoid double work.