Around the rear wheel is just about useless, unless it's around the rear rim (not just the spokes, naturally) inside the rear triangle - then it's just about as secure as around both the wheel and the frame. The only downside to just getting the rear wheel inside the triangle is that a thief might not realize that it's actually secure, and yank on it hard enough to bend your rim. But I suppose that getting your bike damaged is a danger any time you lock it in public.
