There are a few threads around that discuss this issue. I think early on a couple of people bought extra screws, most people neglected to mention anything about them, so I presume you should be OK.

Check out the resources at the bottom of the wiki for the Service manual. In it Nokia say to dispose of them - probably to avoid the heads from stripping at a later date.

Please report back on completion - I have to replace my digitizer in the next fortnight or so.

Oh you might want to do the usb epoxy fix while you have your device stripped, or if you're brave and capable the solder fix as shown here