For hand piecing, traditionally seams were pressed to one side. This strengthens the seam. When seams are pressed open, there is more stress on the single thread that is used for hand piecing.
For machine piecing, pressing seams open is considered fine because machine piecing is stronger than hand piecing. The only issue with this used to be that batting could "beard" through an open seam. This is often not a problem anymore with the battings that are needlepunched, although it can still be a problem with certain battings.