The Genius Bar employees have very wide latitude to waive charges. If you're polite/friendly with them, and if you are obviously competent and able to make their job easier (show up on time or slightly before, don't complain if you have to wait, have all the debugging steps done, be able to assist with simple debugging steps, HAVE THE MACHINE FULLY BACKED UP BEFORE ARRIVING, etc.), they're more likely to waive charges.
Another tip for OSX apple machine service: have an "apple:apple" temporary account created (as admin) for them to run tests, rather than having to log them in to your account or give them your password, especially if you use filevault. I don't really know what the equivalent is for iOS -- maybe turn off passcode lock before showing up?
There are lots of repair issues other than cracked screen; I meant the general case. But yes, even with a cracked screen, back your shit up using the cable before going into the store -- it'll save a step for the Genius and make it more likely you get a free screen.
A fresh account is less likely to have random stuff (desktop full of files, random app windows, customized settings, ...), so it's easier for the tech to work with. They usually ask for the username/password, and note it somewhere -- if it's the default (apple/apple), it's easier for you (you don't need to change your account password) and for them (if the note gets lost, or if your password is absurdly long or complex).
With Filevault 1, you'd be safe as a logged-out user even from an admin on the system. With Filevault 2, they went FDE, and assumed systems are dedicated to single users, or that unix file permissions would be the only protection between user accounts, so one admin account can read everything. (I actually use separate drives when I send machines in for repair -- doing this on the MBA is slightly harder and requires a service tech agreement with Apple).
This applies even more so if you have something like PGP disk or Trend or Wingate or whatever which makes booting more complex.
Another tip for OSX apple machine service: have an "apple:apple" temporary account created (as admin) for them to run tests, rather than having to log them in to your account or give them your password, especially if you use filevault. I don't really know what the equivalent is for iOS -- maybe turn off passcode lock before showing up?