The importance of ground heat transfer will depend on the relative efficiency of the rest of your building and how many surfaces are in contact with the ground.

ASHRAE perceives that enough heat is lost through foundations to warrant insulation levels for slabs and below-grade walls in Standard 90.1. In my understanding (correct me if I'm wrong), few engineers actually simulate the foundations of buildings to claim energy savings for higher levels of insulation. Two reasons for this:

- People assume that foundation heat loss is negligible (without providing much evidence for the claim), and therefore higher levels of insulation won't significantly impact energy savings.
- As you said, the multi-dimensional calculations are too computationally intensive to include in standard energy analyses.

People claiming that ground heat transfer is important are usually (like me) trying to justify the time they spend studying the problem, while those who think it's negligible are looking for an excuse to avoid the calculations. But the truth is, without the tools capable of performing these calculations, we have a hard time estimating how important the problem actually is.

There are few tools which actually simulate the ground domain coupled to the rest of the building simulation. Take a look at this question on modeling slabs for more information.

As to your question about when ground heat transfer is important: Foundation heat loss is stronger near the slab perimeter, so the problem becomes more important for smaller slabs.