Practices probably differ
Most of the agents I know of (including my own) get in touch with the editor and pitch it first, usually over the phone. This often includes 'cold calls,' pitching to editor the agent doesn't know. Miss Snark also says this is her regular practice (and says most agents hate cold calling, but do it anyway).
That doesn't mean that all agents do this, however. For all I know a really high-profile agent might be able to get plenty of attention with a cover letter. But I think pre-submission contact is very common, regardless of how experienced the agent is.
