I am not exaggerating when I say those lines are a flood waiting to happen. They become brittle and corroded with age, and the least movement can cause a crack in a corrugation, or cause it to snap off at the valve. NO plumber I know would touch one. We insist that they be replaced as part of the job.
Now, as to your specific question, the cone washer you need to use with those is a special one, which is actually threaded on the inside, to mate with the twists of the corrugations. This provides both a seal, and a mechanical "grab" so the line does not pull back. A smooth washer cannot seal into the deep corrugations, and does not grab the pipe firmly, so blowout is a real possibility.
I do urge you to replace any and all of those as soon as possible.