Listen to the_jenny, she's got it right (and I learned the hard way too, by trusting a toggle bolt in plaster that held up my spice rack for a whole three days). The plaster just turns to dust behind the wall sometimes, so the toggle has nothing solid to push against. Finding the studs is the only safe bet for something heavy like a cabinet. If the studs are in the wrong spot, you have to open the wall and add a solid wood backing, which is a pain but it's the right way to do it. Trying to skip that step is how you end up with a cabinet full of dishes on your floor.
I used to swear by toggle bolts for everything. Then I had a cabinet pull right out of my plaster wall last year. The problem is plaster can crumble behind the wall, so the toggle doesn't have solid material to grip. Now I always find the studs first and use long screws directly into them. If the studs don't line up, I sister in new wood behind the plaster for a solid anchor. It's more work but way safer for heavy loads.