Dear Michele,
I think you are mixing significant digits and precision: for our latitudes (47...) 5 significant digits means a precision of 0.001 that corresponds to about 100m (too low). To be fair, concerning longitude, 5 significant digits for longitudes above 100° would even be a precision of 1km. So, if you think in terms of significant digits, you would need 7 on longitude and 6 on attitude to reach a 10m precision which is probably enough for our purposes. I am surprised that you had problems with PostGIS because I used it in the past with even higher precision without any problem.
Of course the best way would be to think in terms of precision, where 1e-4 is the minimum we need.
In any case, 5 significant digits cannot be used for building by building applications, at least in Europe.