Exercise Gen. with load when cold outside

When I start my Onan generator, all parts of it are being exercised, including the engine and generator portion. Once I verify that it is capable of producing electricity, I do not bother to keep a big load on it, but that's me. It's worked for decades with all of my generators.
Commercial back-up generators at hospitals and other commercial buildings that I have built only verify that the engine is running and electric is being produced. They do not shut down the power in those buildings to exercise them.