As the owner of a 2.5 - I can tell you it sure can't hurt to have one. I know the heater on my nano comes on semi regularly at night. I'm assuming your tank wont be lit 24/7
I always prefer a heater that I don't need over one I don't have.
BTW 20g's coming along nicely - she's up and running as we speak; hopefully the 175w MH for it arrives early in january.
Andy