most likely, the only actual damage will be to your wallet... from the noise violation tickets your going to get. without a muffler, u r dramatically increasing the chance of cracking an exhaust valve... but thats still not to say that it will for sure happen.
as for u bring up a rejet, i dont understand wat your asking... r u implying that u wouldnt need to rejet the bike if u ran just the stock header, w/out a muffler?? this is def incorrect, as the bike will run like crap w/out the muffler. u may feel that it runs better and that your getting more HP... but most likely its jus the extra noise overshadowing a large loss in low and mid-range torque.
however, if u simply mean using a slip-on muffler w/ no change to the stock head... then ya, dont worry about a rejet. the bike will run slightly better w/ a rejet (even w/ a purely stock exhaust), but unless u really really need the 2 extra HP, dont worry about it.
oh, and the search function is your friend, as this has been talked about quite a bit.
Naked 93' F2...... RIP -- 04/12/07
04' 600RR Black - currently stuck between track and street usage
- Brembo Master, Sato's, & Ohlins out back