Opening Proof Sets?

Depends on the year.

Make sure you wear cotton inspection gloves to keep the coins looking their best.

Older sets in the 1980's you can flex if you try hard enough and crack the corners. The use a dull blade and inch along the crack to get them to open.

On newer sets, there is a very thin seam on the back side of the set, and I usually use a box cutter and find that sweet spot. all it takes is a little prying and the set should crack open. I open sets for resale, since I know there are more single coin collectors out there than set collectors.

The older sets used to crack in half, so a sawing motion in the center would work. Again newer sets, its almost to the back side of the set is where the seam is located.
The 1975 set - those can be a pain to break open.

Just be careful with any sharp instrument.

With my ATB proof sets I grab each end and twist in opposite directions. Comes right open.