Question About PC Fans

Noob here, Im building a grow box and im using pc fans in it, i was wondering, do i need to cover the holes for the fans with something to keep light out?

Like a screen or something? and will it matter if light is leaking in throw the fans?

Probably a dumb question but im still learning.

i used fish tank filter foam over my fans lets loads of air throw but dense's the light .