The answers will vary, according to the situation.
Any number of factors can determine the need for rejetting.
1. Initial jetting, was it rich or lean?
2. Amount of modifications done
3. Type of system installed
4. Existing modifications
The only real answer is to have the exhaust checked with an exhaust gas analyzer. Your local tuning shop ought to be able to perform this test for you. (For a nominal feel of course...)
This will tell you how the bike is running, what areas need worked on, or if it is fine the way it is.
For data taking, you may wish to have this test run before making modifications, and then again after. It is best to do the runs at same shop on same dyno, with similar conditions. This will provide more useful information. (like, did i help or hurt by doing this mod?)
Sooo not to put off the questions, but the only real and correct answer for YOUR case will be to get YOUR bike checked out.