Rankings are not the be-all-end-all in fantasy football drafting. If you took the highest-ranked player each time your pick came around, you might end up with five receivers in your first five rounds. Context on draft day matters. Rankings are a guideline and certainly influence drafting (especially the rankings on the site you're drafting on), but your strategy should be independent of ranking and, most importantly, be adaptable.
You can plan to go zero-RB or robust RB, but there could be a reason to change your strategy based on the players available.