Rib tips generally are attached to the spruce inwales with steel nails, and are frequently badly rusted. If the nails come out easily, that's fine, but if not, you'll do too much damage to the wood while trying to get them out. Leave them alone. When necessary, I've occasionally cut off a steel nail by running a hacksaw blade between rib and inwale. You don't want to take out the inwales. Way too much trouble. If there is minor cracking/damage to rib tips, a few drops of epoxy will take care of that, but if there is more severe rot or damage to the tips, scarf on a new rib tip of 2 or 3 inches. When re-attaching rib tips to inwales, bronze ring nails are often used. If only 3 or 4 inches of new stem tip is required to make repairs, just scarf it on, but if much more of the stem is rotten then a 'beefier' repair as is shown in Stelmok & Thurlow page 166 would be better.
Tom McCloud