grommets for elec cct backing boxes

Open generally - otherwise getting the cables through is a pain. All you are seeking to do is protect the cable from damage from the sharp edges of the box knockout - open does that and are cheaper and easier to insert to boot.

Open are quicker to use. Although being softer can push out more easily. Were you thinking of flame retardation?

TFT

Although being softer can push out more easily.

It's in the back of my mind that I picked up (?2 or 3 years ago) something about backing boxes needing to be completely sealed, including cable entry holes.

Probably misunderstood.

Thanks for the responses.

As they are simply folded metal, "completely sealed" doesn't happen. There are cracks along the corners. And the holes where the tabs for the screws are pushed round. I'd fit a blind grommit to an open, but unused hole though.