Newton's rings are most prevalent in humid climates, and also when positive contact pressure isn't fully maintained. IME it seems more prevalent with some papers than others. They are caused by any small, even microscopic, gap between the negative and the glass, whether air or moisture. With regular glass, a felt or foam backing and good pressure like in a contact frame goes a long way to eliminating them, but not always. With the frosted glass they are eliminated by the scattering of the light, removing the "harmonic" in the gaps that cause them. Exposure time is generally not a problem with contact printing, even with the slowest of paper, so the minor loss of transmission isn't generally anything to notice. I printed with regular glass for a long time, and never thought I had a problem. One humid summer they appeared in droves. I switched to the frosted no-glare glass and they disappeared. Careful examination of earlier prints after the fact revealed earlier, but nearly unapparent manifestations of the same issue. I personally won't use any other kind of glass now.
My 8x10 spring backed frames have 2mm glass, sourced from a framer. Larger glass for larger prints I had etched at a glass shop, and is 3/8ths inch. I don't know how many mm that is, I live in Utah.