A real answer to this would require empirical benchmarking. And yet, even then, you couldn't really count on the answer for long, because it's not a property of dplyr that is actively managed or guaranteed.
Warning: I am speculating now! I would guess that if your data is of a size where this affects performance in a noticeable way, you should be investigating more options than "dplyr: should I pre-sort or not?" and considering much more dramatic changes to the workflow.