If you want the bed to be raised, then topping it off would be a good idea; otherwise, it's not much different than a planting bed at ground level, even though the soil type likely differs due to the amendments. It also depends on the crop you want to grow, but since fall vegetable planting typically doesn't involve deep-rooted crops like carrots (where the extra bed depth is helpful), it might not matter too much.

You can learn more on our Growing Vegetables in Raised Beds page, in the section "filling raised beds" that discusses organic matter content and buying topsoil. You don't want to make the bed too rich in organic matter by having it be pure (or mostly) compost, so the addition of some topsoil might be good, unless you opt to use soil from below the bed or somewhere else in the yard (examples included on that page).

Miri