I've never heard that in relation to pelargonium. Deadheading is just plain good sense. It allows the energy to go where it is needed more than a wilting flower.

Roses, on the other hand, are removed in the bud stage to set up larger blooms.

I dead head when there are two or three flowers left on the stem for the same reason as Caroldiane

Not sure about the size of the flowers, but I have noticed that dead heading causes many pellies to bloom more profusely. All of mine do that. So, from my experience, removing the flowers doesn't cause the plant to grow at all. Just makes it want to produce more flowers, rather than less.

JME