I'v been told that walking with a heavy foot-fall will deter snakes. Is this really true?
As with any wild animals, different species and different individuals are going to respond differently in different situations.
Some snakes are extra-ordinarily docile, and no amount of heavy-stomping will disturb them, unless you stomp on them.
So, to answer the original question, walking with heavy footfall is neither foolproof, nor sustainable for long hikes: resulting in very sore feet at best, and stress-fractures at worst.
Best is to keep an eye on your surroundings, and be careful not to place your foot somewhere you can't see what might be lurking around it - ie, stepping over a log, walking beside an overhanging rock, in deep grass, etc.
Stepping on a snake is an obvious no-no, but even passing within striking distance can result in an attempt depending on the disposition of the particular animal.