Depends on how steep, of course, but if in doubt, better to have too low a gear than not low enough a gear. On steep enough a hill, that could be a huge difference in gearing. As for the three tooth increments near the end of the cassette, 24/21 = 1.14, and 17/15 = 1.13. It's really not much worse than the 17-15 jump in many 9-speed cassettes.