This seems plausibly more reliable than either "avoid all outside influences" (and >hope your uninfluenced decisions are best) or "expose yourself to particular >random priming" (and hope that a randomly influenced decision is best or >harmless).

Given that we don't seem to have much idea what input is likely to prime us to think about what (if I see the letters 'ABC', do I think about US broadcasters, childhood education or matrix multiplication?), it seems unlikely that 'Avoid all outside influences' is a plausible strategy. Is there really going to be nothing in my room which affects my judgment on matters of importance? Indeed, even if I lock myself in a dark room, maybe this primes me to think about ghost stories, or astronomy, or photography? I think wherever you are, you're probably subjecting yourself to some priming effects.

Given that it's pretty hard to avoid all stimuli, I think the 'average your guesses' technique might well be necessary - think about your decision in as many contexts as possible and average your conclusions - if not, you'll never know what influences are lurking in your supposedly neutral environment.