I think as long as a single border width is no larger than your block size it will look proportional with no issues. I'm also of the belief that if you start with your smallest width border and work your way out it would be the most pleasing to the eye.
When I shop for fabrics for a project at the LQS, I usually try to line up the bolts of fabric for the borders in the (rough) widths that I'm planning to sew. If you have various fabrics that you are considering using, fold them to the various widths you are thinking..line them up with your top and see if they are pleasing to your eye. You might need to adjust some colors/widths but folding and lining up with the top might save lots of sewing/ripping. Just a thought.