You have to look really closely at the shrinkwrap. Even the smallest tears or damage will downgrade it.
It can be worth doing for 'good' titles, that is, rare sealed stuff or common in demand stuff.
How long will you keep it? In the long long term (10+ years?), my belief is that grading will be worth it for nearly anything that will grade 90 and up. But if you plan to make a quick flip, grading hasn't caught on universally for video games, so you may be wasting your time and money.
If I was a huge fan of a game and I had it mint/sealed, regardless of where value was going, I'd consider having it graded.
If you are using your last dollar to grade something, don't bother. Grading is a luxury item at this point.
If throwing down $50-$100 for an acrylic case sounds like an awesome investment, then it's always time to have your sealed item graded.