The advice from makai is the best here. I worked in a position supporting network managers at Air National Guard bases around the world. I found that the units that had the most problems were the ones who messed with things the most. The books I have on exchange all recommend basically the same thing. If you're not having problems, don't use the diagnostic and repair tools.
Running a manual defrag to regain unused space is a boondoggle unless you've deleted a large number of mailboxes and have a particular need to regain the disk space. Unless you're using the server for other applications or as a file server, you don't really need to regain the unused space, especially since the IS is going to grow anyway.
If you read up on these tools you'll find that they were not intended to be used for routine maintenance. The Exchange databases are generally very stable. My experience has been that most problems are due to external causes, such as improperly configured antivirus programs quarrantining transaction log files.
I'd like to recommend you get a copy of Exchange Server 5.5 Unleashed from SAMS publishing. Another good reference is Exchange 5.5 in a Nutshell from O'Reilly.
Also, since you're not experienced with these tools, if you ever find yourself in a situation where you think it's necessary to use them, do so only after consulting with someone who is very experienced with them. If you're in a time crunch, call Microsoft. the cost of the call is cheaper than your job.
Good luck,
Bob