Almost every corner walls in the house are a little larger than 90 degrees. Not much of a problem until you lay tile or butt a bathroom vanity against a corner or install base and crown moldings.
Even when the corners are at 90 degrees with new sheet rock but by the time you lay paper and mud the corners its a little over 90 degrees. I've been doing little adjustments when doing bathroom installations or installing moldings but sure would nice to have walls at 90 degrees.