Hello Tran,
It depends on how different the power difference amount to.
The results are not necessarily equal but with respect to a certain condition of the subject (e.g. deep sleeping/wakefulness) or the action the subject is doing (e.g. relaxing/keeping high concentration) the power in the various bands should be similar among channels unless the subject is highly using a particular cerebral area because of the action he's doing.
Before proceeding with PSD computation be sure of filtering correctly the signal and keep only the frequencies you need.
Moreover, look for artifacts that can affect PSD and proceed with artifact removal if it is necessary
