If you are going to attempt to use this tactic, the most important thing to consider is that your offer must be within the range of reasonableness. "Shock 'em with your first offer" is a good sound-bite, but make sure not to actually shock them. If the number that you propose is outside the range of reasonableness, an experienced negotiator will "wipe it off the table" and re-anchor.

For example, if you are negotiating your salary for a first-time software engineering gig and open with $500k, the other side is likely to say something like, "There is absolutely no way we can do that. We would like to offer you $75k." If you're offer gets wiped off the table, your anchor has absolutely no power in the negotiation.

So go high (or low, if you're on the other side of the table), but don't be too extreme.