Is the best defense a good buying offense? Lockheed Martin(NYSE: LMT) announced a 23-million share repurchase plan, that, if carried out, would consume 5% of the company's outstanding stock. With stocks testing multi-year lows, it's only natural for companies to put their idle cash where their shares are. But that's not Lockheed. Defense contractors have been strong performers over the past year, with Lockheed leading the way.

No, the more typical buybacks have been coming from the likes of Nathan's Famous(Nasdaq: NATH). The hot-dog specialist has gotten its buns kicked into penny stock territory, so last night's announcement of a modest million share repurchase makes sense. If no one's buying in, and the vault is flush with spare greenbacks from the good old days of taking swings at the stock-offering piñata, why not earmark some of that money to help support the stock?

Stuck data-storage bellwether EMC(NYSE: EMC) has $5.5 billion in cash, so even the massive 250-million stock buyback it announced last week won't dig too deeply into the petty cash till. However, investors need to approach these repurchases prudently. While it's typically a good sign to see the company investing in itself, let's restate the obvious: If a stock is in the dumps, why should we see management as a stock market visionary?

Want proof?

"The purchase program announced today reflects our belief that EMC common stock is currently an attractive investment opportunity for EMC," said the company's Executive Chairman Mike Ruettgers.

When did he say this? Back on May 9, 2001, when the company announced a 50-million share buyback. The stock closed at $40.98 that day and has given back 90% of its value since last year's repurchase announcement.

So, tread cautiously with the flurry of buyback press releases you're seeing lately. While it might be a prudent move for public companies to show a little faith and try to lead by example, just make sure you know what you're following.