It used to be that traders rewarded or punished Apple’s shares right after its earnings releases based not on the sales it reported for the past quarter, but on what the company said about the quarter to come. Apple tends to “guide conservatively,” in the jargon of the trade, which Wall Street often interpreted as a disappointment.

But that pattern has been broken in recent quarters. In July, the stock popped in after-hours trading despite earnings guidance that was a whopping 16% below the Street’s expectations. And in October, the stock tanked despite guidance that was 4% above consensus.

It turned out that Apple in July was sending a signal about the September quarter  — the one that didn’t include the expected release of a new iPhone — that Wall Street chose to ignore. For the December quarter, Apple told investors to expect earnings of $9.30 a share on sales of $37 billion, which for Apple was unusually optimistic. (Wall Street, of course, is expecting Apple to handily beat both its guidance and the Street’s published consensus for Q1 2012 — earnings of $10.08 on sales of $38.85 billion.)

What kind of signal will Apple send today for the March quarter? On average, according to Piper Jaffray’s Gene Munster, Apple guides 10% below the Street’s consensus on earnings and 2% below on revenue. Assuming that pattern holds, Apple’s guidance this afternoon should look something like this:

• Revenue: $31.3 billion (2% below the Street’s $31.9 billion consensus)
• Earnings per share: $7.20 (10% below the Street’s $8.00)

Any deviation from that pattern will be closely watched.

Below the fold: Munster’s spreadsheets recording five and half years of revenue and earnings guidance.