Greece has once again dominated the headlines this week as the European Union continues to search for the perfect solution to an imperfect problem. This still didn't stop dozens of companies from approaching new 52-week highs. For optimists, these rallies may seem like a dream come true. For skeptics like me, they're opportunities to see whether stocks trading near their 52-week highs have actually earned their current valuations.

Keep in mind that some companies deserve their current valuations. Take Goodrich (NYSE: GR), for example, which is trading near an all-time high following a $16.5 billion buyout from United Technologies (NYSE: UTX).

Still, other companies might deserve a kick in the pants. Here’s a look at three companies that could be worth selling.

What a gas
Today we're going to start off with a two-for-one special: Crosstex Energy (Nasdaq: XTXI) and Crosstex Energy LP (Nasdaq: XTEX). Confused yet? Allow me to explain.

Crosstex operates as two separate entities: a partnership and a corporation. The partnership (XTEX) operates a midstream natural gas business that includes 3,000 miles of pipeline and nine processing plants. The corporation (XTXI) owns the 2% general partner interest and a 25% limited partner interest in the partnership, along incentive distribution rights that allow it to share in the partnership's distributions when they exceed certain levels.

What these two operations do share in common is a recent, steady stream of quarterly net losses. While both entities are currently cash-flow positive, history has shown that Crosstex has struggled to control its costs. With the corporation's shares trading at nearly 275 times forward earnings and future losses expected for the partnership, this is one natural gas pipeline that could be full of hot air.

Give 'em the pink slip
Advisory Board
(Nasdaq: ABCO) might seem like an odd choice as the company is regularly turning a profit and growing revenue steadily by double-digit percentages. But not all appears well with this consulting company.

The particular concern I have relates to the company's steady decline in gross margins since 2005. In that time, gross margin has fallen from 58.8% to just 45.8% over the past 12 months. This trend has also shrunk operating margins from 25% to just 8%. In short, while revenue has doubled since 2005, the company's profit per share has actually contracted. This isn't exactly encouraging news for a company richly valued at 27 times forward earnings and almost 18 times cash flow. Compare this to Accenture (NYSE: ACN), which has steadily increased its operating margin over the past decade and trades at just 13 times forward earnings, and you can see why it may be time to fire this stock from your portfolio.

The emperor's robe
Apparently KiOR (Nasdaq: KIOR) has figured out the same formula that most biotechnology companies have figured out; namely, how to amass a multibillion-dollar valuation despite having no product or revenue stream.

KiOR, a renewable fuels company with a technology that converts biomass into renewable crude oil, has taken investors' hopes and dreams and run with them ... first out the door, then over the hill, and finally onto another planet. Investors have a nasty habit of blindly buying into alternative energy sectors only to be disappointed by the long technological and social acceptance delays attached with alternative forms of energy. We’ve seen it happen with ethanol, hybrid vehicles, and even solar panels.

As of right now, nothing leads me to believe that KiOR, which doesn’t even have its business up and running yet, is anywhere near being worthy of a $2 billion valuation. If anyone else can justify a reason to buy in here, please be my guest and state your case in the comments section below.

Foolish roundup
It's foolish (with a small “f”) to pay a premium for a company or a technology that you could just as easily get cheaper by looking at competitors or being patient. Sometimes patience is the key to a winning portfolio and these three companies could soon be very close to facing some very impatient investors.

What's your take on these three stocks: Are they sells or belles? Share your thoughts in the comments section below and consider adding Crosstex Energy, Advisory Board, and KiOR to your watchlist to keep up on the latest news with each company.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.