If you undo the bolts and replace them you will usually need to get the head skimmed, if you are just tightening up the bolts that are there then it's worth a try and a skim is not required.
I don't get it ? why would the head need to be skimmed if the head bolts get replaced ? if this is the case then obviously I will try using the current ones because if the head had to be skimmed if using new HB's then obviously I'd just replace the head gasket.
Also observe the correct tightening sequence - this has been designed to prevent warping when doing up head bolts.
FYI - There is also a difference between torque on a wet bolt (oiled) or dry one so don't get caught out by this little anomaly.