Here's what I use for help; although I usually set them all at 5/5 and 5/5 and work from there, or just leave them stock.
Damper Bound/Rebound
This is probably one of the most useful setting for tuning your car. You can control almost all balance, oversteer, understeer, or any other quirks from here. Let it be known that the following info was taken from a post by "Exit Speed" (Thank You soooooo Much) in the Granturismo.com Settings Workshop Forum. Also with these, like springs, youll want to make them stiffer for flat tracks and softer for bumpy ones. Though Id mess with the springs first to correct bounce and contact. Remember: Bound = Shock going up / Rebound = Shock coming back down.
Loose into turns
Soften Rear Rebound / Stiffen Front Bound
Loose out of turns (Increase oversteer when throttle is released in mid-corner)
Soften Front Rebound / Stiffen Rear Bound
Push into turns
Stiffen Rear Rebound / Soften Front Bound
Push out of turns
Stiffen Front Rebound / Soften Rear Bound