For about 10-years, I've used a trick that has eliminated Newton's rings so far. I went to the glass store and had them cut two sheets of anti-glare picture frame glass to a size that fits on my scanner. I put the slightly frosted side of the glass towards the negative - one beneath the negative and one on top. The bottom glass raises the negative to the proper height for focus, the top sheet flattens the negative. I don't use the carriers that came with my scanner.
juan