Is there a border on your quilt in addition? If so just keep in mind you'll have a little bit more overhang with the additional blocks AND a border. I don't think there is a "rule of thumb" per say - but usually when you make a quilt to fit a bed size you do want a little overhang...so I think you're fine! What matters more is that you make it so that it makes you happy - you have to sleep under it after all!