Probably not as much as Mark Wyner from Campaign Monitor.

We usually test our emails in Outlook, Yahoo, Gmail, Hotmail, and we also occasionally use Lotus Notes and Mozilla Thunderbird. While Mark tests all these, he goes much further by testing in different operating systems (mac, PC & Linux) as well as three different handhelds.

If you ever wanted to know exactly how in-depth testing can get, Check out the full article.