* Will the rule improve the game?
* Will my players like it?
Page 1 of the DMG
The D&D rules help you and the other players have a good time, but the rules aren't in charge. You're the DM, and you are in charge of the game.
and a bonus one about advantage:
The DM can also decide that circumstances influence a roll in one direction or the other and grant advantage or impose disadvantage as a result.
600+ hours of EA and still having a blast!