If your feeder design is what I think it is, a screen cover on top of it will save you a lot of grief later. It's not an issue of how often you peek in. That can't be helped.
The problem is that tops rarely fit nice and snug and are subject to warping. It doesn't take much of an opening for snooping bees to find their way under the top and into the feeder. Next thing you know, you take a peek and find 500 dead bees in your feeder. Effectively turning it into a bee trap.
You can use any kind of screen you wish. It doesn't take anything special. Put it on nice and tight with enough staples that it doesn't have any places it pops up that a bee can get under it. They are quite the little industrious pillagers whens there's sugar to be had.
"Success is not final, failure is not fatal: it is the courage to continue that counts." Winston Churchill