I almost pulled my heads to do a valve job but my mechanic had a method of pumping compressed air into the cylinders with the valves closed. He said if the valves were leaky then there would be an audible hiss. In my case he decided worn rings was the culprit. Years ago it was sort of a risk to do valves and not the rings as well because the firmly seating valves increased compression and worn rings allowed "more" blowby. Therefore doing a valve job only kinda didn't work well.