I always use OFFSET(Fluke), REL(Keithley), and NULL(HP) after shorting out the leads to measure DC, but I haven't used the AC function other than to measure the main, and this was with a hand held.
When I measure DC Volt, I short both of the leads and press the OFFSET button for accuracy. When I measure the 300 Volt in a freshly NIST Calibrated HP 3457A, it displays 0.29 volt whether I short the leads or not.
If I NULL this when I measure 40V AC, its effect is huge.
Anyways, the calibrator said he doesn't use it. And the manual does not go into specifics of how or when to use this function.
[EDIT] Even though manuals say "The OFFSET feature may be used in all functions." - Fluke 8840A manual page 2-10 & "You can use the NULL operation on any measurement
function (DC voltage, AC current, and so on)." - HP3457A manual page 2-25. Obviously it shouldn't be used in case of AC because the OFFSET's purpose was to take out the leads's effect. In case of AC V or I, shorting doesn't shrink the number. The number has something to do with inside the meter. For example offsetting 0.3V to measure 32.5V is huge.