I live in the Seattle area and even here, the weather is very unpredictable ("if you don't like the weather, wait 20 minutes"). On the coast, weather prediction is even more difficult.
I've been there in all seasons; getting 1 or 2 sunsets out of 4 or 5 days is easy, the catch is provided you pick the right 4 or 5 days. In a good year.
It's been said that beautiful women travel in groups--beautiful weather seems to follow this same pattern. Best chance of sunset may be in late summer when the air is driest, but Hoh is at it's brownest at this time as well.
I've found the most spectacular light to be in the early fall and late spring seasons, as the weather systems transitioning from summer to winter/winter to summer patterns. The unstable air creates the most spectacular cloudscapes for the setting sun, giving all the sky interest you could wish for. The drawback is its easy to go 4 or 5 days without seeing a sunset!
Simple answer, it's a crapshoot. If we have a good, warm, dry summer, you'll be able to come the same time as you did last year or later and do very well. If we have a cool, wet summer, (like last year's) it will be up to the photography gods whether you'll catch the magic rays.
HTH,