I took a quilt class about this. She showed the difference in sewing all in the same direction and then sewing each row the opposite direction. It was amazing at the difference.

It's natural for a strip set to have some curve, nothing you can do about it. When I have long strips to sew more then two together I baste glue them first then sew. It helps more then any other way I have tried. It's hard to press the seam flat with no lap over the seams. I spend more time pressing then I do sewing.

I am a firm believer in Ironing and NOT pressing. I always, ALWAYS see tucks when pressing. I think the real trick is to slide the iron in the direction the threads run and not in a diagonal direction. Before you set the strip set aside to do the next one, look to see if it straight. You can iron it back into position if necessary, unless of course your seams are wobbly. And yes, it helps to sew in opposite directions when adding each strip.

when sewing strips together it is important to switch ends with each addition...so you put the first 2 together, then start the third strip at the end you ended on (flip-flop one end to the next) this helps keep your strips from (bowing) also, it is important to not (iron) your seams but Press them...no back and forth...lift, place, lift the iron. but most of the time the distortion is caused from starting at the same end with each new addition. really need to flip-flop then trim-square up one end to start cutting your squares.