The nice thing about the Google Chrome's auto-update feature is that almost all users are running the latest version of the software. This is really important for the web developers, who no longer have to worry that a large number of the users haven't upgraded from Chrome 2 or Chrome 3. It's not necessary to test a site in the multiple versions of the Google Chrome and it's safe to use features added in the latest version of the browser.
Gmail's help center lists the supported browsers and includes the minimum version number required next to each browser, except Google Chrome.
Unfortunately, even if Chrome installs the latest version, you can't use it until you restart the browser. Google started to add a little orange dot next to the wrench icon and show messages like: "Old school's not cool. Google Chrome is woefully out of date because it hasn't crashed or restarted in a while. Restart Chrome to apply update."