I try to document as I go. It sometimes takes twice the effort to do it later. Things like: what IP did I give that database server? Add it to a wiki or something while you configure it rather than wait till later and have to run a nslookup or whatever to find it again so you can add it. Once it is documented it is a lot easier to configure clients properly because again you don't have to look up the ip from a console every time your setting up the connection.

Next up why this is good: hardware/software installed on clients leaving the lab. Heck half the time you'll forget what you called that system. You end up having to go to the persons office to get the computer name then fiddle around with it to see what all you installed on it and document it etc. Much easier to add the computer name/location and stuff on it when you actually have it in the lab to play with. Most times the user won't know you spent an extra 10 min with their new machine before giving it to them anyways.