Like most respondants we only reboot our servers when neccessary. However as an aside this means you need a scheduled job to cycle your sql logs regularly (we do once a week, but we keep more than the default 6 past logs). The first time you try and check out the log file of a system that's been backing up transaction logs every 15 minutes for the last 6 months without cycling the sql log you'll remember this point... That's "DBCC Errorlog" for ref.