There are times when the three things you list would be important, and there are times when any or all of those three things might destroy important information and be very much the wrong thing to do. It is not practical for anyone to detail all of the circumstances under which one might want to undertake or refrain from undertaking any of those three operations.
If, though, you were to describe a particular context -- signal characteristics and desired output -- then we might be able to explain the use of those techniques in that context.
