We discussed this at a guild meeting earlier this week. Two people who long arm quilts said that seams pressed open are more likely to distort or even pop when loaded, given the stress on them. LA's pull those tops pretty taut.
Personally, I find it takes too long as it requires a lot more pinning to line the seams up. When pressed to the side, nesting naturally occurs. Without that little ridge to bump up against, pinning becomes needed. Most of the tops I sew have lots and lots of small pieces - 2-1/2" is pretty much the biggest I use - so it is a lot of seams to pin, slowing me down.