According to Motorola's FAQ here, it's best to reboot once a day:
Originally Posted by JohnnytheK
Your phone has capabilities that make it almost as powerful as a computer, so like a computer, for optimal performance, give it a fresh start by rebooting it once a day. Turn it off for one minute, then restart it. Rebooting refreshes your phone software and your connection to the nearest cell tower. It can also help extend your battery life.
I usually do that, but I also use a program called Fast Reboot which, using a one touch widget, "DOES NOT ACTUALLY REBOOT YOUR PHONE, only the apps & processes." If things seem a little sluggish, it usually gets them moving again. The program does not require root.