I have heard, and even subscribed, to that theory myself as a self-confessed former whacker. I have subsequently found it not to be true, and to be a validation for the whacking, which *may* actually resolve issues when the vice and bench are nowhere near as stout as the whacker would like to think.
In other words, whacking may be necessary with an improper vice/bench setup, and in fact *may* overcome that shortcoming, but with a stout bench and vice the leverage wins every time. As does, by the way, setting up the upper in the correct orientation to take advantage of pulling into the bench instead of lifting the bench.
IME and all that jazz.