I have replaced my lines with stainless. The lines I replaced were rusted for a reason. Brake fluid will absorb water out of the air. Most people don't flush their brake lines every few years, so the stainless is good insurance. Same with all the alcohol and additives they put in gas these days.
Stainless fuel lines are a good idea, and easy to replace the 5/16" with 3/8" for better fuel flow.