It depends on what software you're using, but in general, I'd say that you're safe. For example, in Nero the meter
includes the track/session data, so the 5mb aren't at all necessary. It'll also pop up a dialog as you initialise the burn process if the session is too large for the disk, before any writing occurs. There's also the option to do a test run if you're not satisfied. Without knowing any particulars, that's the best advice I can give you ie. try a test run first and note any error messages (if any) and if successful, proceed as usual.
{Edit: I assumed that you got the size of the session from within the app and not from disk. Overburning can be risky with audio CDs, especially if your friend has an older player.}