I use gutter guard over the eggcrate on my overflows to keep small fish out of the overflows. Bigger fish it's not a worry. I find my FW tank some fish at first are small enough to still fit through, but it's better than nothing.
Problem with adding stuff water flows through (eggcrate, gutter guard or anything for that matter) is that it becomes a place algae can grow where grazers can't get at it. In my 110g cube I have to pour boiling water over the overflow eggcrate once every 2-3 months or so just to keep them clear.
Parker has a good point, something I sort of learned the hard way with my first eel - sometimes it's better to let them get into the overflow instead of somewhere else. I had had my eel tank locked down so that there was no way he could get into the overflow - so instead he ended up getting out of the tank some other way. Had he just gone into the overflow or sump, it would have been .. possibly less fatal for him.
So there could be something to the idea of just putting strainers on the pipes and "just deal with it" when a fish inevitably finds his way in there. FWIW, that is my plan for my 280g - just eggcrate for the overflow, and strainers for the standpipes, and a net nearby.
