The conventional wisdom I've heard is that spending close to your limit and paying the balance off in full each month will often lead to the credit card company increasing your credit limit automatically. I assumed this was because they expect that given an increase, you'll spend more and earn them more merchant fees. In this comment, Dilip gave another (related?) reason, in that your credit card company may do this to preempt you moving to another company, who presumably would give you a higher limit from the beginning.
Obviously, the effect of utilization on your credit score means you're probably better off asking for an increase instead of having high levels of utilization month after month in the hope your limit increases, but is this an actual phenomenon? Is it a standard practice for credit card companies to raise the credit limits of "deadbeats" who spend close to their limits?
I've never had this happen personally, because the one time my credit limit has been raised without my prompting, my utilization was low. On my very first credit card, I had high utilization (around 90%) and a low limit, but I had to ask the credit card company to raise my limit; it never happened automatically. Now, my utilization is well within the 1-20% range, so in the event my credit limit is raised without my asking, it still doesn't tell me anything about the "conventional wisdom."