A bit of a tricky one to answer. For beginners I suggest pressing to one side for ease of butting seams together for matching. Also this was advised in the past because there was a theory that batting/wadding would 'beard' through open seams (I've never had this problem!), and that they were less likely to come undone as there was no reinforcing stitch at the beginning and end of the seam. As you know I always do a back stitch at each end of my seam so it does not come undone. I therefore I can press to one side OR open with no fear of the seam coming un-stitched.

I personally prefer to press my seams open as this is less bulky and makes machine quilting easier because there are no lumpy seams for the machine foot to stick on - which results in small quilting stitches over the seam. Matching seams requires care and secure pinning if you do press your seams open.

Really I think you have to decide what is best for you/the project. Which way gives you the best matching results and does the pressing matter to you during the quilting process?