While the perception may be that any repairs recommended by the dealer are nothing more than a fishing expedition to relieve the car owner of their money, this is seldom the case.
It is common practice while a car is on the rack to look it over for other problems. This not only drums up work for the mechanic and shop but also lets the car owner know of other problems that may exist. Any mechanic who does not do this is lax in my opinion.
The vast majority of cars that come in to a shop WILL need other things, whether the car owner knows it or not.
As to the repairs listed, it is quite possible to have leaking valve cover gaskets (or anything else) and not leave spots. Oil has a tendency to blow back and there simply may not be enough there to leave a puddle, or even a spot.
And the use of the word "seal" is a bit misleading. A DOJ on a Subaru is Double-Offset Joint so this means a halfshaft boot, and the passenger front side boot is the one most prone to failure.