Leaves are a good amendment for your soil. As mentioned, I'd avoid black walnut if possible otherwise use whatever you have or can get. The more broken down the leaves are by mowing or shredding the faster they can decompose in the soil for next year.
I add some leaves in the fall and till them in. I have never used leaves to cover the soil nor have I grown a cover crop for the winter. I suppose if you live in an area that exposes your soil to constant winds then it is a good idea to grow a cover crop (something with roots vs just leaves sitting there that would just blow away) to stop erosion. I would not till the leaves into the soil in the spring therefore I would not recommend putting them on the garden as a cover over the winter.
Leaves are carbon and require nitrogen to break down. By turning those leaves into the soil in the spring they will take (steal) nitrogen that your plants require. This is why I don't and would advise against covering the garden with leaves over the winter and tilling them in come spring.
I get dump truck loads of shredded leaves delivered by a local lawn care guy who is happy to have a place nearby to dump them. I let them sit and breakdown so after 2-3 years they look more like soil than anything else. I use them all summer in the compost bins and I add just 1-2" of the shredded leaves before rototilling. When totally decomposed, I spread them for mulch in the summer and will also till them in come fall.
I used to think that you can't have too much organic material in the soil but I don't buy that anymore. I add manure and leaves because they are good amendments but within reason. I used to add too much manure and leaves thinking I was just doing a super job (I add pretty good soil to start with) but more is not always better. You can have too much organic matter in the soil (!) as I found out in a very comprehensive soil test. I had double the rate of organic material I should have had and I had issues with too much salts or phosphates (I forget) from all the cow manure! I now only add manure to the garden every few years, about 1-2" vs the 4-5" I was adding. I add some shredded leaves each fall but again not a lot.
So unless your soil is in terrible condition I'd be moderate with the addition of organic material. If you are building quality soil from junk then I do think you can heap it on but in a garden with good soil a little goes a long way. If you think of this as a natural process, how many leaves or manure in a pasture would a given area receive naturally? I am totally organic so I believe in organic methods, it is not that I am decrying them. I just caution that sometimes too much of a good thing is too much!
Tom
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