This is more of a parenting philosophy question. My philosophy is that babies either will sleep anywhere, or they won't sleep anywhere. So don't attempt to completely black out their room as you may be setting them up for unrealistic needs later in life.

The fact that babies happily fall asleep in cars or buggies in the middle of the day just goes to show that complete dark is not a requirement.

It's not so important when they're babies. But once they hit the toddler years blackout curtains are great. The darkness is an easy signifier that it's time to sleep.

But it also depends on the room. Will the sun ever shine on the crib/bed? Does the room get a lot of sunlight?