when I first started quilting, I was told to "press to the darker side". Then a few years ago, I saw a gadget demo'd at a qulit show, called a "strip stick". (
http://www.thestripstick.com/ ) To me, this is the best tool since the rotary cutter!! My blocks not only lay much flatter, but they tend to be more accurate (I am very prone to IRON rather than PRESS, which of course leads to distortion.)
With that said, it is still very much a matter of personal choice. Unlike some of the other posters here, I have not had any problems with the batting bearding through the seams. And I have not run into "the quilt police" as someone else did.
Just my 2 cents worth.