You also have to consider the UCI rules of bike set up. If you are setting up a TT bike, your nose of your saddle can not be less than 5cm behind the center of the Bottom bracket. So that limits TT positions. However, if you are just doing Tri's, it wouldn't matter (though check up on the rules on whatever the Triathlon governing body is).

But for most people, you'd never come across a UCI official that would actually measure dimensions on the bike to make sure its legal.