I was in your situation 6 years ago and decided to bring new soil first. In my opinion, improving the soil is the best thing you can do to your garden. I ordered and spread many truck loads of half top soil and half compost. Topsoils Northwest will mix it for you before delivery.
I made berms so in addition to having great results (everything I planted grew like crazy), there was an added interest in varying landscape.
If I couldn't do it, I would add compost on top after planting like you suggest. It may be better to dig it in but I just spread a thick layer on the top so it's pretty and weeds don't grow. I used to think that doing it in the spring is better because you can cover the leaves and other mess left over after winter and it looks good. But now, I'm thinking that doing it earlier that is in the fall is better because the compost protects the plants; I read somewhere that the ground won't freeze that quickly if you have compost. I wouldn't worry about the compost washing off with the rains. Are you're planting on a slope?

That's my 2 cents; more experienced gardeners may correct me :)

Add compost now, so the soil microbes can work on it over fall - winter - spring.

If it washes away, over the soil surface, then you need to do something about your topography before you can have a garden anyway 'cause your whole garden is going to wash away.

It won't wash through the soil like fertilizer does. You would be correct about not fertilizing now because of the rain, but organic matter like compost and organic fertilizers won't wash through the soil so quickly and they need 'curing' time in the soil, which they get over winter.

With new construction, the contractor scrapes off all the topsoil because a) it won't hold the weight of a building, you want subsoil for that; and b) he can sell the topsoil for a good price.

It was my understanding that King County requires contractors to set aside the topsoil and organic layer, and replace them when done with their building activities, but I don't live or work there so that was just something I thought I saw somewhere.

You should check out the "mulch, soil" forum on Garden web. They have MANY suggestions for improving soil and I have used some with great success.

Soil activity decreases during winter, when the ground becomes cold and wet. By spring conditions are the poorest for planting of the whole year. Spring planting should be limited to tender plants like annual flowers and vegetables, that would die if planted in fall. (There is also of course the problem of the selection in garden centers not being what it is at the preferred planting time in fall what it is during the spring frenzy). Look for good topsoil and buy that, dump it on the ground and plant. Mulch with wood chips after planting.

Here is a link that might be useful: Horticultural Myths