An interesting thing came up during my last session (D&D 4e). There's another wizard in the party besides me and we had two battles in the same area (mutated dogs, then our Longtooth Shifter Ranger character's mutated pack mates). I was unable to use my spells (or so I thought) because the ranger would've taken friendly fire from my area attacks because I believed that you had to have a creature as the target square for the center of the burst attacks, so I had to hold off on using them. Meanwhile, the other wizard was targeting his spells on empty squares near the enemies and catching them in the burst area of the attacks. Our DM (a rather liberal DM who lets us use as Many Daily Spells as we want, rather than the 2 per day cap) was allowing this, but I was wondering what the official ruling on this is.
So my question is this: What is the official rule on targeting with burst attacks (and blast attacks too, I suppose)? Must the center of the burst be on an enemy, or can it target an empty square? Thanks.