Holly,
I would definitely get a good idea of where your full-sun, semi-shade and full-shade areas are before you decide on a location for your raised beds. The general rule is that, for a veggie garden, you are better off finding a full-sun area. If you have to settle with a semi-shade spot, then try to find one that at least gets the morning sun and then shade in the evening. If you live in the southern part of Alabama and have hot summers (I live in southern Louisiana so we definitely have this problem), then semi-shade in the afternoon might be a good thing and may even extend your growing season some for certain items like tomatoes which can sometimes have pest problems in the hot, humid summer of the south.
As far as the trees go, I think that the type of tree would be a good indicator of how dense the leaves would be (i.e. the more dense, the less sunlight let through). For instance, we barely have any trees but the ones we do have are Pecan. Well, they aren't too dense. So, I would think that it would be a safe bet to place a garden under it's shade if I had too. That said, perhaps if you know what types of trees you have and don't know how dense they will be once they have leaves, then you could do a little research.
Also keep in mind that you will want it as close to your water source as possible. It will be much easier that way.
I have raised beds myself (my preferred method of gardening). I know that they can sometimes be an investment, especially if you are filling them with dirt and if you build them with more long-lasting materials such as treated lumber. Therefore, I highly recommend that you choose your spot carefully before setting up.
Just my 2 cents.....
God bless and happy gardening!
Julie