So glad you asked this question, Anita. I answered thinking about ironing brand new fabric. But based on some of the answers, I'm wondering if you meant blocks/seamed pieces.
I set the seam from the wrong side & then press open from the right side, sliding my iron from the low side of the seam over to the high side of the seam. Then, I double check the back to make sure I didn't accidentally press in any creases. However, sometimes even when I initially pressed everything the right direction, along the way, it ends up flipping & I usually don't catch it until it's too late. I know some members here use Water Soluble glue to tack down the seams in the proper direction. I might try that this next time so they don't end up flip-flopping on me. Or maybe someone will post some ideas about how to better prevent this from happening. I use little to no starch or steam when pressing. I switched to a Clover II mini iron because I have nerve damage to my fingers & kept accidentally burning them with my full sized iron when pressing pieces since I cannot feel hot or cold. No blisters since switching to the mini iron so hopefully I can keep working with that. If I need a bit of steam, I have a spray bottle of water with which I can spritz my fabric.
In the event you were asking about applique (which is the one thing I can actually do pretty well), I always press on the right side of the fabric. For turned edge, I will tack down a bit with my iron & painted on starch on the folded "seam" of the applique piece (still the right side of the fabric, but the part that's flipped over & caught between the applique & the background fabric). For fused applique I used a pressing cloth & press always from the right side of both fabrics.
For needle turn, you don't press at all -- which is why I love it so much! 
A huge THANK YOU everyone who's responded/will respond and to Anita for asking this question. I always learn so much from everyone on this Board!