Well, there must be something wrong with my VPS then. I just found a client w/ 3.5GB of usage out of an allocated 750MB.

I got no notification. He got no notification. The account continued to work as normal. Worst of all, cPanel and WHM did not report this (until I ran the fixquotas script myself, manually).
I'm just tired of not knowing when a user is over quota. This seems like auto notification should be a standard option with cPanel, no?
And of course it would be the client's fault if they go over quota. They should keep a leash on their content, and be sure not to go over quota.