Generally speaking, we model all buildings, structures, roads .. ad nauseum ... at their real world coordinates, for example UTM (Universal Trans Meridian). In other words, nothing would ever be modeled at 0,0,0. However, the model's basepoint (INSBASE) would most certainly be 0,0,0.
Agreed, we, too, place things at absolute coordinates withthe file's insbase set to 0,0,0 for XREFing purposes.
There are, however, exceptions. Such as individual equipment models, which are built around 0,0,0 match their insertion point. Example: a distillation tower is modled such that the center of the bottom o fht base support ring is at 0,0,0. The piece is then XREF'd into whatever files it's needed at it's absolute coordinate location.