I think in geothermal collectors the hot end is a lot hotter than the surface at all times. So it isn't a form of heat storage where you pump heat down in summer and retrieve it in winter. The heat flow is always upwards to the surface.
Oh, wait, I misread the question.
Yes, you want it to be a source in winter and a sink in summer.
But my comment still applies. Too near the surface and the ground will be cold in winter when you want to extract heat from it, and warm in summer when you're trying to pump heat into it.
So you want to go deep enough for the temperature to stay constant all year round. That way it's colder than the surface in summer and warmer than the surface in winter.