to the question.. yes FCOM does say not to alter winds at different waypoints if a difference of less than 30 degrees and 10 kts...(but i also do the 10 degree / 10 kts thing)
the other thing to keep in mind is that the IRS measure actual winds upto 200 nm ahead of the a/c... so really only inserting winds on longer legs makes a difference.
..and the difference it makes is like one other said...accurate predictions and better descent planning, optimum alts....etc