Hi Hardpole,
Na, not optional.
But I'm on freeride gear, sail the ocean and just do this for fun also. I got RealSpeed when it was free, but have since coughed up the dough.
Your GPS will give you your max etc, but will be worth alot more to you with software like RealSpeed.
RealSpeed is an Aussie program with top service.
But only you will know if it's worth it for you I guess

Cheers,
Bob.