You have to tailor the air pressure to each application. Tire design, static weight, transfer, suspension, shocks, "hit", etc. all play a part. If you're not getting a full-width footprint at higher pressures and it's unstable at lower pressures, it may help to go to a wider rim.

I have Hoosier's 9" lightweight radial slicks on 10" rims and found that 20 - 21 lbs works best, despite a little extra wear in the center. But it's contacting all the way across the tread. When the rubber off the tread was fraying over the edge towards the sidewall too much, I added air. Some Stock class guys run down to 14! (I swap the slicks from side to side after every race to help even up the wear. I've never had equal side-to-side wear if I didn't.)

A wider rim will allow more air pressure, which stabilizes the sidewall and prolongs its life in addition to helping the car on the top end.