If you have a variable wind (adopts Eccles voice and reads, "Winds, light and variable") you might want o be a little conservative and use a 10 knot headwind on all legs.
At work we have some semi-completed flight plans. We work out the average wind component for the route and round up to the next figure. Our plans have fuel and time figures for +20, 0, -20, -40, -60, and -80 knots.