No, you do not need a level site, mine is not level. But I do suggest leveling the beds as it makes it a lot easier to water them & prevent erosion problems in heavy rain areas.

Your new potager sounds like a great project! I hope you'll post pictures. Everyone likes before and after pictures, not only to see the dramatic change, but to insprire those of us who are fairly new to the forum :)

I have a similar situation to yours--a slight slope, but very visible. Ours is in our side yard, but fully visible from many of the main rooms of the house, as well as the outdoor entertaining spaces.
In our case, we're choosing to level the site because we're making it quite a formal potager with wooden raised beds, gravel paths, etc. I don't have a photo of the area, which is currently buried under about 18" of snow, so I will try to describe it: I'm putting in 8 4x4 beds with 4' gravel paths between (think of a grid but with the center bed missing; that space will have an armillary). The eastern side extends into a slope which then slopes up much more sharply a few feet later, ending in an old stone wall with woods beyond. We are planning to level about a 5' by 28' area for the gravel path, and use the soil we removed to terrace a small section immediately above (to the east of) the gravel path. This terrace will probably include a small stacked stone wall of two or three courses using found stone from the property. That will tie in nicely with the ancient stone wall about 15-20' further up the slope, and I can use that terrace to plant blueberries, which will be a beautiful addition to the potager.

How steep and long is the hill? You might be able to think of it as an opportunity to construct some nice masonry. In other words, stone walls are beautiful - why not build one to level out your new potager? If the hill is not too big, you might even be able to do the excavation by hand. Cut back the hill, level the area in front of it, then construct your wall. It could be informal, using fieldstones gleaned from your property, or it could be a fancy mortared wall which would be expensive but nice, or you could use some type of decorative concrete blocks. This all makes it sound easy, which it isn't... building ANY type of stone wall is a lot of work. But since you said aesthetics is important, it may well be worth it.

-Diggity

Here is a link that might be useful: Garden Imperative