That does work remarkably well in bull phases. As do positive options expirations (at least over the past 7-8 years).
It should be noted that the May top occurred on the first of the month on a significant gap and it's been steady selling ever since. The indicies are pretty much masking the retracements seen in alot of the bellweather names, many of which topped out several months ago. There's no doubt, in my mind, that the market's shallow pullback is mainly due to the fact that nobody believes Bernanke won't unleash QE3 if things get materially worse (or just if enough time has passed). It's despicable, but this is the reality of 21st century finance.