nothing awful will happen either way! for me it depends on what kind of block i am doing.Pressing seams to the dark is something from hand quilting,to make the seam stronger.
"Pressing seams open creates less bulk on the back side of the quilt top. It also helps you sew a true 1/4-inch seam, because less fabric becomes tied-up in the pressover.
Like many other quilting tasks, there are no absolute "right" or "wrong" ways to sew a seam. What's important are the techniques that work for you. Try pressing seams open to see if you like the results."
Opinion by Mabry Benson:
http://www.straw.com/equilters/libra...SeamsOpen.html
Please do what works for you!