Most USB mice work as soon as you plug them into your Maceven two-button, scroll-wheel mice. Using System Preferences, you can even program your spare mouse buttons to invoke cool features like Expos and the Dashboard.
That's not to say, however, that you shouldn't install your mouse's driver software. If your mouse came with such software (or if you find it on the manufacturer's Web site), you may well find that your mouse learns a few new tricksmaking its "back" and "forward" buttons work properly in Safari, for example. Otherwise, a shareware program like USB Overdrive ($20, from www.usboverdrive.com) can unlock those features.