Two of my raised beds are on my lawn, and two are on an old garden site (my back yard isn't that big). When I laid them down, I first covered the bottom of both with landscape fabric to make a barrier between the beds and the ground underneath. You don't have to till the ground underneath at all, unless you are planning on incorporating it in to your system. The beauty of the raised bed system is that you can place the beds on concrete if you want. The growing medium is in the bed, not underneath. Here's a photo from last year.
Grant

Thanks for the reply Grant. I guess I am overthinking it a bit. You touched on something when you said don't till the ground unless you are incorporating it into your system. Are there any good reasons to incorporate the ground underneath if I already have a 12" high bed (e.g. for drainage, root vegetables, etc). Thanks!

Based on what I read (first year SFG), at 12" you should be fine for even root veggies in the bed only, no lower amendment / tilling required.

At 6", you might want to till, but it seems easier to just build up (that's what I'm doing -- building a taller box).

Good luck.