Since I'm going to be replacing the valve guide seals on my '91 CTD, I thought it was a great time to upgrade to 60# HD valve springs, but should I replace all 12 of the valve springs, or only the exhaust valve springs?

I understand that only the exhaust springs are needed for full effect from an exhaust brake, but all 12 should be replaced if I intend to spin the engine over 3500 rpm. Is that correct?