^what he said. If the head gasket is on it's last hurrah you'll notice the engine will be running rough (unless it goes Chernobyl on you without warning) It is actually possible to replace valve stem seals without removing the head - but it is always a good opportunity to tear it down and do a general health check on it if you are going to replace the stem seals. A little port work, a valve cut, weld up any coolant galleries that look corroded and give it a skim to reface it - and it'll be good to go for years.