Originally Posted by Justin_Luhrs

Not doing the valves will not kill you for 15-25K miles more use, most likely.

Valve related problems can include:

Surfaces of the valves being too thin and as a result getting very hot which can cause them to burn and give you poor (no) compression, also wear in the guides can cause mis-seating which can also cause a valve to burn. The place where they valves meet the head is critical, as valves reach unreal temperatures, and if they cannot dissapate heat through the valve seat evenly, they will DIE.

Deposits on the back of a valve can cause performance issues, as the air/fuel mixture does not travel easily or most completely into the combustion chamber as a result of them.

From a German mechanical or engineering standpoint, at your mileage it would be heresy not to do everything. Your car after all probably needed it a long time ago, even before your overheat, for purposes of durability and efficency.

But in reality, you can probably slide. As duke said, you'll need to pull the head before you know any of it for sure.