That's my procedure anyway. What you tell your testifying expert is discoverable, so whatever you put in writing should be perfectly worded to put your point of view in the best light. But e-mail encourages quick, sloppy writing, and is not always conducive to organized, thoughtful exchanges. What's worse, when you use e-mail, you're encouraging your expert to reply by e-mail, and you can be sure he's not reviewing his response from a lawyer's point of view.
Perhaps I'm old school and paranoid, but I won't even send a letter to an expert. I just attach a business card to whatever it is (such as medical records) I'm sending. All other communication is by telephone.