Not just rear-end sag, but also (and perhaps more importantly) front-end rise. As you mentioned, front-end rise reflects the unloading of the wheels that do all of the steering most of the braking, and this is an invitation to disaster.
At risk of starting a wildfire, I'll also say that simply measuring rear-end sag encourages people to install helper springs and air bags, to level out the vehicle. However, simply raising the rear end of the vehicle an inch or two does nothing to unload the rear suspension (tires, wheel bearings, shocks, etc), leaving it potentially overloaded even though "it looks good". And more importantly, it does nothing to re-load the front end, replacing the weight that is missing there.
Bill
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