Revelle German U Boat, 1/72 flood intakes

I've used a Dremel wire brush on inside for any removable areas, then "finished" with xacto knife.

I suppose it depends on the scale, however I think its way better to open them. You can drill or thin and punch. I plan on trying both methods as they are both probably equally effective.

I prefer to thin out the plastic from behind (with careful control of the dremel speed to avoid melting the plastic and using a backlight to determine the depth of the cut) with dremel circular cutter bits when I can...

Works great for improving portholes and general thining out of edges too: