From the standpoint of salvation, it doesn't matter if it was a T-shaped cross, a t-shaped cross, "stauros", or a plain old tree. To Christians, what matters is not the precise manner of death, but the resurrection that followed three days later.
The only significance of the Watchtower's insistance on "stauros" is that it's their way of saying "We're right, you're wrong, and Jehovah's gonna get you unless you agree with us!"