Are tuition subsidies taxables for research staff?
I am not a Lawyer or Accountant, and if you really worry, you should ask one of those or at the very least talk to human resources at your university. 

But, based on my reading of the relevant law, part (d) [usually referred to, I guess, as Section 117-4] specifically states that if you are an employee of a university, then tuition reduction for you to take classes at said university should not count toward your gross income when calculating taxes. 

The US code does not distinguish between in-state / out-of-state tuition. 

Caveat: your local state laws may have different interpretation of "gross income".