If you switch the top and bottom of a duplex receptacle, it becomes a lighting outlet and would not meet the requirements on spacing for receptacle outlets as specified in 210.52(A)(1)
So...common practice is to switch half of the receptacle outlet in a room to meet the requirements of both 210.52(A)(1) and 210.70(A)(1)
It is not required to be split,but if you don't you must also locate a "Live" receptacle outlet next to the switched lighting outlet to meet the spacing requirements.
My practice is to switch the top half so when the homeowner plugs in a cordless phone with those little square power transformers it does not cover the other half of the receptacle.
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