This works, and most of us have probably done it this way or seen it done this way at some point (I've done it back as a teenager and didn't know better). The sole reason, as far as I am aware, of grounding the last clamp away from the battery instead of on the negative terminal itself is a combination of:

* The last clamp will close the circuit and may produce a spark
* Batteries can generate explosive gasses so you want to keep sparks away from the battery

By clamping +/+ and -/- terminals, you're closing the circuit and it works, but any potential spark occurs right at the battery. Most times you can get away with it, but it's that rare time that you'll regret it.