Besides the aforementioned issue of cost, there are a few other items that you might want to consider.
How tall of plants do you plan on growing? Even a 1 or 2 foot tall plant in a raised bed to chest level might cause you problems because you would have to reach up to tend to the plant. Even if you can reach it without a step, it can cause strain on your back to be reaching up all of the time.
Also think about how far out you would have to reach. A shorter bed allows you to grab over a plant, where as a chest high bed would require you to reach in between plants. Unless you have a very narrow bed or large spacing between plants, this could cause problems.
Also think about how you will add soil. Over time your soil will settle so you will most likely need to add more soil. Lifting a bag of soil to chest height is a lot more strain on your back then lifting it into a shorter bed. A related note is how you will till up the dirt. Tall beds will only give you the option to till it by hand with a small hand shovel, where as a shorter bed you could still use other methods.
You will also want to consider how much sun the plants will get. This may or may not be an issue, but depending on your surroundings different height beds will get more or less sun. You might also run into problems with wind. Again this may or may not be an issue, but depending on your surroundings, it could be. Just make sure you take this into account.
Overall I would say, you should consider various levels depending on the height of plants, but the highest I would consider is waist level.