Valve springs are one of the hardest worked, but least understood parts on any engine. And while a lot of engine parts, even the cylinder heads, might get replaced during an engine’s lifetime, the valve springs from the old heads will most like just get transplanted onto the new ones.
Make no mistake, though: valve springs, like every other stock engine part, weaken over time. That means if you’re planning on making any kind of engine modification that’s going to increase RPMs, pushing stock valve springs beyond the point they were designed to function is guaranteed to result in parts failure.
The solution is easy: install new valve springs. There’s more to valve spring selection, however than just getting the biggest, stiffest springs you can find. You have to select the one that matches your application; and more importantly, they have to work in harmony with all the other parts on the top end of your engine.