As a casual display for decoration, I think they look most natural displayed closed, showing the front of the match only.

As a collection, my opinion is they need to be open and visible on both sides. Not practical in a frame, but you can use plastic pages designed for matches and put only one match to a pocket (not back to back).

If they are old and historic, and again part of your collection, my opinion is matches should remain in full books. If there are matches missing, I usually treat it as a 'cover only' and remove the rest so the cover lies flat. If the match is a feature with matches missing and I have no full book, I'd stay with the partial book to show it is a feature.

Sulphur match heads are not a fire hazard unless you have them loose and they can contact a striker where the oxidizer is. Yes they smell like sulphur but that's the only problem. I mean they ARE matches [g]!