I wouldn't say 20% at least for general riding. Cruising around on the flats I would be surprised if it made much if any difference. But when the road points up, the steeper it gets the more important it is to be able to pull up. I was doing 3x12 low-cadence strength endurance intervals (seated climbing) today and the only way to muscle the cranks around is to apply the force around the entire stroke.

Also it's very useful having a power meter because I can actually see and quantify the forces involved. For example when starting from a dead stop, the torques involved are greater than my body weight, which means that I am getting contribution from the upstroke as well as the downstroke. For the seated climbing, 230W at 50rpm up a 10% grade worked out to about 480lb-in of torque, or about 70lbs of force. Seated, so no bodyweight contribution. The upstroke balances the downstroke to assist in power generation.