Interesting thread. Guys who are building all the morale boosters for their lower morale worlds should be careful: the cutoff (based on the PQ multiplier) usually means the extra people you can support are effectively worthless. Better to reduce the pop, or allow those planets to have a morale that stops the pop growing. I NEVER build multimedia center, vr center, spin control, secret police etc for morale - I've never seen a planet that made a profit from it.
Going for 100 morale makes a lot of sense when you want the pop to grow: once it reaches its cost-effective maximum, you want to leave it alone (until you increase the PQ).
If you are showing a surplus and want to be able to spend it, higher population (higher morale) is something you need. They will spend and provide you with more taxes. Though i never build VR center either.
Furthermore higher pop. will help your influence. I believe these choices to depend on the type of game you play.