In large part it depends on the story you're trying to tell. If you're trying to tell, say, a morality play about racism or euthanasia or overpopulation, what's the point of making the aliens eight-armed crystalline monotremes or whatever? That's not the point of the story. If anything, making the aliens too exotic would just be distracting.
If, on the other hand, you're doing a story about the challenges of communicating with truly exotic lifeforms . . . well, then you probably can't
settle for a few bumps on the forehead.
It's all about throwing your time and resources at what ultimately serves that particular story.