Keng, according to my experience, it is required to rebuild the Service Target everytime you change a value, this includes the Business Entity as well. This is probably because there are workflows built in the back end when you create/modify the Service Target. So rebuilding the Service Target modify the filter qualification and actions accordingly.
Geetha is correct however with SLM 7.5 or later a service target only needs to be rebuilt when the changes made to the definition require new workflow to be created. So if you change the Description field, for example, and save the service target you will not receive the message that the service target needs to be rebuilt.