I'm no fan of market timing ... but now's the time to time the market and start selling. Or at least, that's what the old saw, "Sell in May and go away," would have you believe.

Normally, I wouldn't give a second thought to that kind of old "investing wisdom," but I happened to catch Bloomberg columnist John Dorfman's article on the subject earlier this week. According to Dorfman, Ned Davis Research has shown that since 1952, the average six-month return for the S&P 500 was 6.36% between November and April, and just 1.58% between May and October.

Sounds pretty convincing to me. Let's start selling!

... Just kidding
If that were Dorfman's conclusion, I probably would have moved on faster than Cramer in one of his Lightning Rounds. Instead, Dorfman suggested that May is a great time to comb through your portfolio looking for stocks that you'd rather not own any more. For long-term investors, holding through the summer doldrums should be no big deal -- particularly if you can collect dividends -- but why continue to hold stocks you don't like through this annual slump?

The trick, of course, lies in figuring out what stocks might be prime for the chopping block. Here are three situations to watch for as you review your portfolio.

1. Dividend cuts
Dividend cuts have calmed down since the worst of the financial meltdown, but that doesn't mean they've disappeared. Here are a few companies that have slashed payouts this year:

Company

Date

Dividend Action

Current Annualized Dividend per Share

Valero (NYSE: VLO)

1/27/10

Cut dividend by 67%

$0.20

Tesoro (NYSE: TSO)

2/2/10

Suspended dividend indefinitely

$0

Home Properties

2/18/10

Reduced dividend by 13%

$0.58

Source: Company filings.

All three of these companies' dividend reductions reflect the pressure currently clamping down on their respective industries. The vice-like squeeze that's crimped refining margins leaves refiners Valero and Tesoro far less free cash to kick back to investors.

A dividend cut may not be reason enough to dump a stock from your portfolio, but it's often at least a yellow flag, and a good reason to revisit your investment thesis.

2. Management shakeup
A change in management isn't necessarily a bad thing, either. But when the captain changes, you better make sure you're OK with the direction in which the new arrival will be steering the ship. Here are a few companies that have put a fresh face behind the wheel this year:

Company

Date

New CEO

CEO's Previous Role

Novartis (NYSE: NVS)

1/26/10

Joe Jimenez

Pharmaceutical division head, Novartis

CIT Group

2/7/10

John Thain

CEO, Merrill Lynch

Bristol-Myers Squibb (NYSE: BMY)

3/2/10

Lamberto Andreotti

President and COO, Bristol-Myers Squibb

Source: Company filings.

In Bristol-Myers's case, the change looks pretty positive. Former CEO James Cornelius was a temporary pick, taking over in 2006 after a scandal forced out Peter Dolan. From the looks of it, Andreotti will keep the company on Cornelius's path, while adding a sense of stability to the CEO spot.

On the other hand, it may be worth keeping an eye on Novartis. Jimenez is a veteran of the packaged-goods industry, having spent his career with companies including H.J. Heinz and Clorox. There's talk that his experience could help the company contain costs, but that's a pretty significant change from the usual pharma CEO with a long history in the industry.

3. Mergers and acquisitions
A few companies have been incredibly successful at building their businesses through M&A. But all too often, big M&A deals end up more of a cause for concern than celebration. Here are a few significant deals thus far in 2010:

Company

Announcement Date

Deal

Deal Size

AIG (NYSE: AIG)

3/1/10

AIG sold its AIA Group subsidiary to Prudential PLC (NYSE: PUK)

$35.5 billion

CenturyTel

4/22/10

CenturyTel agreed to merge with Qwest (NYSE: Q)

$22.4 billion

Hertz

4/26/10

Hertz agreed to acquire Dollar Thrifty

$1.2 billion

Source: Company filings.

AIG really hasn't had much of a choice when it comes to its divestiture strategy. But investors have to be at least a little concerned about what will be left behind for them once the company finishes breaking off its successful, saleable businesses.

Meanwhile, the Hertz-Dollar Thrifty deal could give Hertz a leg up in the very competitive auto rental market, but a competing offer may yet be coming from rival Avis Budget Group. This is exactly the kind of dealmaking struggle that can distract management from the company's day-to-day business, and increase the chances that Hertz overpays for its quarry.

Cut with care
To be clear, none of the above are "sell" recommendations -- just illustrations of the kind of stocks worth putting under the microscope right now. Since we're not market timers, there's no need to jump to sell just because of the threat of a long, cold summer. But if there are stocks in your portfolio that have lost their good looks, now may be a good time to say au revoir.

Are there any stocks in your portfolio that you're thinking about cutting? Scroll down to the comments section and share your thoughts.

Editor's note: A previous version of this article listed Prudential Financial (NYSE: PRU) as the acquirer of AIG's AIA business. The Fool regrets the error.

Novartis is a Motley Fool Global Gains recommendation. Try any of our Foolish newsletter services free for 30 days. The Motley Fool has a disclosure policy.

Fool contributor Matt Koppenheffer does not own shares of any of the companies mentioned. You can check out what Matt is keeping an eye on by visiting his CAPS portfolio, or you can follow Matt on Twitter @KoppTheFool or on his RSS feed. The Fool's disclosure policy assures you that no Wookiees were harmed in the making of this article.