Depends who you ask.
As for me, I would say yes. And I would say that for several reasons. 1. It will greatly reduce evaporation. 2. It will keep your fish (and shrimp) where they belong. 3. It will prevent your light from accidentally falling into the tank and getting fried. Not to mention you void any warranty on a light by not using a glass top. I've always used glass tops and will continue to do so, despite the fact that they certainly block a small portion of light and they don't look quite right to some people. And others will disagree for these and other reasons.
In short, make your own decision.