Lone pair of electrons on carbon

[DariushS]

When is it correct to draw a lone pair of electrons on a carbon atom in a molecule when drawing resonance structures? Is it correct as long as the carbon atom fits the X=Y-Z* model? The "X" atom and "Y" atom are double bonded and the * denotes the "Z" atom, which is single bonded to the "Y" atom, has either an empty orbital, orbital with one electron, or a lone pair. In carbon's case I believe it would usually be an empty orbital.

Edited June 23, 2014 by DariushS