In my personal opinion it is better to have the same information on the 3 altimeters. The stby altimeter is, as its name says, a stand by tool and therefore you must have the chance to cross check the main altimeters with the same reference datum, i.e 1013 or QNH. I know that many people use to set the local QNH on the stby during cruise as soon as they know the destination airport QNH. That can be useful in case of an emergency descent, but anyway if you perform an emergency descent you will ask anyway local QNH for the area you are overflying to comply with GRID MORA. The point is that I prefer (IMHO of course) to have the same data on all the sources of informations (altimeters) so when I change one thing, I change everything. I prefer to forget std 1013 on the stby, and landing with QNH set on main altmts that lading with 1013 on main altimeters and the QNH on the stby. Last thing, some aircraft may have the pressurization screwed up by incorrect stby altimeter setting.