I use my pocket knife because I know it's sharp. If the knife is sharp and the person holding it knows anything about knives it's fairly safe...it's when the knife is dull and/or wielded by a careless person that accidents happen.

When I don't have my knife handy, kitchen shears work fine too.

My trick is to cut the 'blister' part and not where the two halves are joined as the blister has the thinnest plastic and the edges are thick and heat fused together. This also allows me to cut through only one later of plastic instead of having to cut both around the edge.