It's always anoying to me when i play a hero with more than four abilities, because the ultimate ability is not on "R" by default in those cases.
I suggest to always put the ultimate ability at the fourth place in the hud.
For example Shadow Fiend has only four active abilities: By default: Q W E Y, because R and T are reserved for the passive abilities.
Puck: He has an ability to teleport himself to his orb. It has the hotkey R and so pushes the ultimate to T. Why not swap these two?