I find that my cadence range just varies more in cross. I still hit over 100rpm's on flats. Having said that it is still important to be able to accelerate when you are stuck in a big gear in a bad spot. But you should still be shifting. I had the chance to do our weekly cross practice with some pro's and on a perfectly level course that required no braking they were shifting way more than I was. They were always in the optimal gearing and that clearly helped keep them going faster.
so yes to being able to ride low RPM's but try to shift more to avoid it.