If you actually live in NY like your profile says, you can vote for whomever you want without worrying about indirectly helping Romney because NY is going Obama, period.
That being said, your reasoning is flawed. The reason a vote for Jill Stein would "take a vote away" from Obama is because it is presumed that people who vote for Jill Stein would vote for Obama if she weren't running. If you prefer Obama over Romney and vote for Gary Johnson, you are just as much "taking a vote" away from Obama as if you wrote in Tom Wallisch.
I voted Jill Stein because she was on the ballot, but I liked Rocky Anderson better. Obama will win though.