The Netherlands is also really flat, but atmosphere believe me affects the results.
That means, atmosphere only can be neglected if the area covers few Km2 (1km x 1km or 2km x 2km) or big deformation is expected (Earthquake events, or huge subsidence expected)
From my personal experience atmosphere matters, mainly when deformation to measure is small. If you measure deformation in the Mexico city center, atmosphere effects can be neglected.
Cheers