Using the nearly-obsolete cone wrench for spreading peanut butter is a noble application of the bike-shop icon. Using it to spread brake pistons, however, is an indication of beater status. Unlike the purpose-designed Park Tool Brake Piston Press, its edges are pad-damaging sharp and you're bound to bend the handle.
Not to mention, peanut-oil residue on pads or brake dust in your lunch isn't going to lead to good things down the road. The Brake Piston Press is designed specifically to slide in-between pads without compromising integrity so reinstalling wheels and calipers is a struggle-free affair. It's constructed from steel, with a rubber handle that's offset for easy, ergonomic use.