I planted mine in 16 inch beds, but I'm not sure it is necessary. All my beds are 16 inches. hehe.

I do know they like sturdy roots to not topple over. So I'd go the deeper just to be safe.

Here is a link that might be useful: Sinfonian's garden adventure!

6" is fine, you just need to tie some strings in a criss-cross pattern throughout the corn, near the middle of each stalk, once they get up to a good size.You will, of course - need something outside of the bed to tie the many strings to.This will catch them,if they try to tip over from the wind.