There's a trick I've learned that has come in quite handy when I've come across screw's with strange tip's like on old gaming system's. I take an old bic pen and pull the ink out. Then I use a lighter to carefully melt the end of the bic pen just enough to be mold-able (Not too much or you could risk melting the plastic housing of whatever you are taking screws out of) and then while it is hot I press the heated end of the plastic down onto the screw making sure it is as straight as possible and holding it there for 2-3 minutes, until it's cooled back to room temperature, with enough pressure that it allows the plastic to mold to the tip of the screw. If done properly you now have a home made screw driver lol. I figure a bic pen would be too big for the screws on the iphone, but maybe you could find something else, like a cocktail spear or something. I recommend trying this out on something inexpensive first before trying on the iphone though. I used this tons on like nintendo 64's and gamecube's and stuff an...
Generally you should only need to push it in so that you can't see any of the ribbon past the line. Sometimes the line will be marked a little off and so wont even go in that far. I would just push it in as far as it will go without putting any real force to it. That ribbon is very easy to tear. Sorry no pics, I don't have a camera atm that I can connect to my computer :P