Why do you expect the decision made in a default state to be more accurate than if it's modified by an unknown factor? If you don't expect to be primed in any particular direction, you may as well be swayed towards the right choice. Priming occurs even when you think of something yourself, so the advice could as well be extended to suggesting not to think about anything, especially about things relevant to the decision at hand, since those things are likely to influence the decision most. When any shadow can influence the outcome, your decision is probably no good to start with, and adding some unexpected noise won't hurt that much.