Just replaced 3 of the Wilcox-Crittenden bronze tapered valves after 28 years. There was no corrosion, just some wear of the tapered plug and bore preventing a good seal, in turn causing a slow drip from them. They still all turned OK.
There are still 6 of the WC tapered valves working fine. These are the less used ones so have been less subject to wear of the bore.
We'll see how the Groco replacements fare.