I think what many people don't understand is the the WWDC (World Wide Developer's Conference) is centered mainly around software development. There doesn't have to be any sort of update to any hardware items during the conference. Yes, in the past there have been, but that doesn't make it a guarantee that it will happen.
This is also a good example of why the waiting game isn't always the best course of action to take if you are looking to get a new Mac.
If you are interested in hardware, then if you are within 2 weeks or so of the MacWorld Expo, then maybe hanging on a little while would be prudent.
If you are interested in software and are a week or two away from the WWDC, then it would be practical to wait.
Any other time of the year it wouldn't make much sense to wait unless you need to wait due to monetary issues.