Deej,
In a perfect world you'd reset the ride height every time you adjust the chain (because the eccentric thingummy bob mucks it up).
But, most of us would probably set it every couple of adjustments, after sprocket changes and maybe a few other things too.
Why do it? It sets the static geometry correctly (before you do sag etc) so basically it sits right with no rider on it and weight is correctly distributed.
Have a look here at how some of us do it now
http://www.section8superbike.com/tech.htm For the manual's how to (This is for the early 748/916 so the actual distance to measure may be a little different)
http://www.chathamclose.com/pics/ride%20height.pdf Bottom of the page is in English.
[Edited on 3-3-2005 by Mr_S]