You don't need to tune cars. You can do almost every race with no tuning.
Heck, that's half my fun. Cars all feel unique before you tune them to all feel "right". I like a little character, adds to the challenge and fun of cars. Some cars plow, some spin, some corner neutrally. Some have long gears, some have short. Some like bumps, some like flat. Some like fast corners, some like slow. Some feel heavy, some feel small.
IMO, it's like removing Eva Mendez's freckle, or reducing J-Lo's booty. It may be closer to "perfect", but it loses it's identity.
Online, however, at any given PP level you'll be dusted by tuned cars. Transmission optimization, LSD tweaks, and some suspension tuning can knock big seconds off of lap times and increase consistency. I keep a few cars at each PP level that I tweak and tune to my heart's content. The rest stay basically stock. Maybe a test pipe and ECU, maybe tires, that's it though.
Cars I love, I'll buy one to keep and one to tune.