Yes, I've read somewhere, that if you are machine piecing, pressing open is fine. But as Lobster, said, and what I read, that when hand piecing you should not press seams open because of weaker seams and migrating batting. And there are always exceptions and better ways to do things depending on what you are making and what results you want.
Originally Posted by Lobster
Don't press your seams open if you are hand-piecing, as there is a reasonably high chance that batting fibres will migrate through the gaps between stitches. Otherwise, go for what suits you.