If you pay for a subway ticket and are sitting, you are allowed to stay sitting while some old lady gets on and has to stand because the seats are full. That doesn't mean a more considerate option doesn't exist.
Pretty much everyone on here is saying you have the right to do it, but it is considerate to A) minimize the level of recline if you can, B) recline slowly and/or give the person behind you a heads up if you need to recline all the way. It never hurts to at least pretend to give a rats ass about your fellow travelers.
Hell, if some disgusting slob that hasn't showered for three days or brushed his teeth gets on board next to you and pulls out a stank ass McDonald's quarter pounder with extra onions, isn't that a little inconsiderate?
The point is there's a lot of real estate between what you have the right to do and being a traveler that shows modest consideration for other travelers. If you have a condition, you need to recline, and pretty much everybody will understand that. But instead you just start off with saying it's your space, you're using it all, you're not saying anything to the person behind you, and people suggesting you do otherwise are pricks.