Replace the stock valve springs ONLY on the Intake Valves. Leave the stock springs on the Exhaust Valves. The Exhaust Valves with stock springs and ER Rockers have been tested to near 7,000 RPM and do not float. Installing heavier springs is only adding to parasitic HP loss and additional valvetrain stress and wear.

High mileage (stock or otherwise) valve springs are not bad by any rationale or by virtue of their age or length of service. In fact, they actually have a higher reliability due to their age and use. This is because springs undergo a type of stress relieving called aging and elevating temperatures increases the rate at which the steel ages. As the springs undergo this stress relieving, they will loose some spring rate or tension. The rate at which they loose their tension decreases asymptotically with time. This means they undergo much of their stress relieving early in their service life and much less later on. If a valve spring is manufactured properly, they will not undergo fatigue failure. I would have higher confidence in springs that have been in service for 100K miles than new springs. The older springs have proven not have any defects in the metal that will lead to a premature failure. These older springs have withstood the test of time. Regardless of the decrease in spring rate, the springs will far out-last the engine in all respects.