This is a late reply, but I just found this forum, so.....
Cydia offers so much for your iPhone. However, installing too many utilities and apps that you dont know what they do (or can do) is not always good.
For example, do not install OpenSSH if you dont plan on accessing your iphone other than using iTunes. OpenSSH can leave your iphone vulnerable to attacks if the default root password is not changed.
If you're a beginner and just want to jailbreak your phone to look for cracked apps and games, then install Installous or AppCake and use those two to install apps from and not Cydia.
I used to install themes and SBSettings for my 3G, but gave up all that on the iPhone4 to avoid problems.
So the answer to your question is: it depends on what you want your iphone to do.
Rule of thumb: install only what you need and not more. Just because you can, it doesnt mean you should.
Hope this helps.