There are holes from which I can hang the springboard upon which my son's mattress will rest in the Graco Lauren Classic Crib we bought from Target. I thought I remembered that you could set it on the top most hole as long as your kid isn't up and walking around, but I'm not certain and the manual that I found online for it doesn't really specify this information.
It's a balance between your child's safety and your own convenience -- you don't want baby to fall out, but you also don't want to bend down farther than needed.
Use the height that best matches this balance. Use the height that prevents baby from climbing out, but err on the safe side because one day you'll be greeted by your kid in the hallway even though you thought he/she was safely stowed away. That's when you know you weren't far enough ahead of the game.