Get a hygrometer. This handy device measures humidity. If the humidity is 80% or less with the lid on, then you don't need to make any holes in it. If it's more, then you may want to add a bit of ventilation, but start small. It's easier to make the hole bigger than to try and cover some of it up! Some hygrometers come attached to a thermometer, which is also handy. You don't want temps to get too high, especially if you have any critters in there!

I just use something to prop up the edge/corner of the lid to allow humidity to escape or to get more ventilation on hot days. Drilling holes seems excessive.

Ah, but some of us are perfectionists, Mr.B! =) We want our covers to fit perfectly, and lay flat! None of this propping-stuff-up-with-unsightly-blocks-or-what-have-you nonsense! =) Besides, my humidity is never too high, even though my tank is basically entirely covered. In fact, I wish I could keep the humidity up! (As I eagerly await my misting system for my birthday. . . ) =)