I have glass lids on my tank, I keep them clean weekly and don't use the plastic light guards on my PC's. I like to have the glass covers for several reasons: 1) I have small kids who like to throw balls, action figures, nerf-stuffs and would hate for anything to end up floating in my tank. 2) I don't have an R.O. unit to refill evaporated water and the glass helps keep the evaporation to a minimum. 3) I have active fish who startle each other and more than once have heard a swoosh, splash and dull "thud" of a startled fish jump up into the glass - gobies are notorious for it. The light penetration may not be as good, but the pay off of having a lid works for me. Up to you. Best of luck.
-Dr Marco:sfish: