That’s the implication of a chart published by Piper Jaffray’s Gene Munster in a note to clients Wednesday. Munster analyzed the movement of Apple share prices just before and just after nearly a dozen Apple milestone events dating back to 2004. What he found was that Apple shares, on average, …

• fell 0.7% the day of the event
• rose 0.4% from the day before to the week after the event
• rose 4.2% from one week before to one week after

In other words, if you wanted to play the odds around next week’s Apple Worldwide Developers Conference, you should have bought Apple (AAPL) two days ago.

A closer look at Munster’s chart, pasted below, reinforces that lesson. The three biggest gains — of 15.1%, 13.3% and 15.9%, respectively — were realized by investors who bought a week before the introduction of the iMac G5, the Intel iMac, and the unveiling of the original iPhone. The biggest losers were those who bought Apple the day before last January’s Macworld hoping to cash in on the MacBook Air. A week later, the value of their investment had fallen nearly 13%.

Here’s that chart (e-mail subscribers, click here):

There are other factors to consider, of course, like the current stock price, the broader market, and the nature of the announcements coming next week.

The investors who trade stock tips on The Mac Observer’s Apple Finance Board have been batting this topic around over the last couple of days. (See Trading around WWDC.) They note that Apple’s stock price, which has been on a roller-coaster ride, is trading perilously close to its 2007 high, which increases the downside risk and might discourage new investments. They worry also that the 3G iPhone may not have enough bells and whistles to impress the Street, or that traders will have difficulty grasping the significance of the SDK (software developers kit). Further complicating matters is the fact that options expire on Friday June 20; trading in advance of expiration could take the wind out of any post-WWDC rally.

“Without talking options,” writes Eric Landstrom, a regular on the board, in summary, “there are three ways to trade WWDC.

1) The long-term investor non-trader, fat and happy average share price of $82 bucks a share who is looking for another opportunity to scale further in. For these people you are looking for a sell off right before or several days after WWDC if any opportunity presents itself at all.

2) The over-crowded who-knew-everybody-else-was-thinking-the-same-thing- fast-money-Jim-Cramer-watching-trader will sell a portion of their position right before WWDC and hope to buy on a dip after WWDC.

3) The I’m-smarter-than-the-cattle-trader is looking to buy the dip #2 traders create and sell into the iHype following WWDC amazing announcements and buy back in a week after iHype subsides.”

Got that?

Of course, it Steve Jobs doesn’t unveil a new iPhone on June 9, all bets are off.