I prefer Fall. By spreading it out and leaving it sit on the soil surface, it breaks down much quicker than in my compost bin. By planting time, it's much easier to blend in than stuff directly from the bin. Your mileage may vary.

I do mine in the fall. It takes a bit for the top dress to make it's way to the root system so if you apply in spring when the plant awakens, it will be early summer by the time the plant begins to show results from the application.

If you apply in the fall, the winter will break down the materials even more and makes things more readily available for the plants in the spring with the rains.

Just my logic and my experience. Also gives you more time to do other things in the spring. Consider that this is something which can be done - in my case - well into November/December before the big snowfalls hit. :O)

Yup, if you have to choose a once a year time, I'd vote for fall, too, for the reasons Tiffy said.

I dig in a lot of compost in the fall, and dig it in over the winter. In the spring I add compost on top as mulch.

Top dressing with compost can be done anytime you have the compost. If that compost is applied to the soil in the fall, as the soil is cooling down and the Soil Food Web is slowing down less will be worked into the soil then if it is appled in the spring, although it will still be there to be worked in by those soil bacteria when it is spring again.
When you have finished compost apply it, do not wait until the "appropriate" time.

Just so we are on the same page, yes compost can be applied at anytime.

I will be purchasing this compost and would like to apply at the time to get the as I mentioned, "biggest bang for the buck".

It seems as though fall would be best as it has time to begin breaking down and by spring ready to be absorbed.

But on the flip side while its breaking down during late fall and early spring will it just seep through, ecspecailly during spring thaw when the roots are barely growing?

I just don't know how quick it will decomposs and if it "remains" in the soil or seeps through.

I am spoiled here I top dress in fall first and again in spring I also have three heaps that can get as big as 4 cubic yards each A heap is used in spring while B heap gets attacked in fall C heap becomes A heap at fall for spring next season and during next spring I start a B heap again and B heap is done being feed the last day of july when C heap starts

Your basic soil type will influence what happens and what you have done with that basic soil type will also. Someone with Lake Michigan beach sand for soil could see, because that is really well drained, the compost and nutrients flow out IF no other organic matter has been added to the soil to aid in filling in the pore spaces that do allow water to flow right through that type of soil. The more tightly binding clay type soils may hold the organic matter, compost, and nutrients in better, but unless that clay is opened up with organic matter those nutrients may not be available to the plants.
It depends on your base soil, but more importantly what does happen will depend on the Soil Food Web in your soil.

I would add expressly what I think is implied by the foregoing that the Fall is sooner than the Spring from this moment, and so the Fall is better.

In other words, there is no reason to wait, which I think, at the end of the day, is your question.