...in their 2nd edition book, K&S have the out and back detours off 101 to La Push and the Hoh Rain Forest as part of their main route. But in their 4th edition, they no longer have them as overnight stops, but just as side trips. So are they worthwhile overnight stops?
Yes, they are worthwhile stops, at least in my opinion.
If you just stick to the main (US 101) route, you will only pass through Olympic National Park at two places, about 15 miles around Lake Crescent, and about 15 miles around Kalaloch Beach. The rest of the route between the Grays Harbor area and Port Angeles go through a mix of private and state/federal forest lands, with a few small towns. The scenery is ho-hum to okay for most of that part, but nothing spectacular, and you won't see the ocean except for that section around Kalaloch. The spots inside the park are way better and more interesting than what you just see on the main road (excepting the portions 101 goes through the park.) I wouldn't do a tour out that way without factoring in the "side trips" to places like La Push, Hoh, Sol Duc, etc.