Well that rule is applicable only for cases where you are instructed to change heading for 5 deegres, for example if controller tells you to turn right 5 deegres, but it is not applicable if ATCO instucts you to fly heading 145!!! It can be very important in airspace where minimum radar vectoring altitudes are different and those 5 deegres can play very important roll in keeping you out of the MRVA which is above your flight altitude at the time!