I would add only well chopped up leaves or ones that have rotted into leaf mulch. Even then I prefer adding them in the fall as they will merge into the soil quickly.

I chop up my leaves very finely with my mower by windrowing them and BACKING up with the mower until they are finely chopped and left in a 6 foot circle...easy to do once you get the hang of it.

I have gotten some beautiful leaf compost from the city, but it contained 10% small rocks...also a bit of plastic though not a lot.

It's good as long as you give a lookover on the leaves first and not just start burying them. Shredding them up (however you can do it) would also be suggested but there have been people burying whole leaves without adverse issues (though one I can think of is that as the leaves decay, you might end up with a depression of sorts, so just be careful walking around there). In your case, you might not walk much on your veggie garden, but for others who might do this in other parts of their yard, this would be a caveat with full-sized leaves.

Also know that their effect may not be fully realized for this season's veggies.

I picked up bags of grass cuttings from some field once. I thought I had found a great treasure but then when I opened the bags, the cuttings were covered with oil. Apparently the big mowing machine that had cut these was either leaking oil or maybe that's how they are. I ended up tossing all of them instead of using them because I wasn't sure how much more oil was in the middle or bottoms of those bags if the top was already like that.

This post was edited by gardenper on Wed, Apr 16, 14 at 13:10

Yes! Leaves have a lot of nutrients that the Soil Food Web will convert into something plants can use albeit slowly. Adding leaves to soil will also improve that soil, making clay soils workable and enabling sandy soils to hold both nutrients and moisture.
I have not found Black Walnut leaves to be a problem in my planting beds, although the trees roots are.

Here is a link that might be useful: Nutrients in tree leaves

As far as Black Walnut leaves go, they fall so much earlier and deteriorate quickly [and get mowed up] that they are not likely to be in leaf bags.