That's not completely true. There are cases where one might want their caster biased more one way than the other, depending on needs and priorities. Going higher (~10-11) gives you more straight ahead stability, but slower response in your steering. Going lower (say ~9 or 8.5) gives you less stability but quicker responding steering. Etc. Its personal preference and use of the car that would dictate for each person where their caster sweet spot would be.