I am building a zen-subtheme which should have 3 breakpoints depending on viewport-size, but none of the layouts has sidebars, there is a small horizontal navigation for all layouts. By default, Zen provides a responsive-sidebars.scss and fixed-width.scss - since i need media-queries on one hand but no fluid sidebars etc. on the other hand, i don't know, what's the best way to go.
Zen 5 has mobile/responsive built-in, and it works really well. I found the conversion from Zen 3 fixed to Zen 5-Responsive fairly easy, and I went with the dev version (Jan 2013). As suggested, you could take out all the sidebars, but they don't really get in the way.
I had to spend some time learning responsive images, in particular responsive background images.