And as they bound him with thongs, Paul said unto the centurion that stood by, Is it lawful for you to scourge a man that is a Roman, and uncondemned?
Note 1 at Ac 22:25: It is very interesting to note that Paul used his Roman citizenship to stop the Romans from beating him (see note 2 at Ac 16:37). This is the opposite of what he did in Philippi (see note 3 at Ac 16:37). It is possible that he simply didn't want another beating, so he demanded his rights; or perhaps this was the leading of the Lord that eventually gave him an all-expense paid trip to Rome (Ac 25:10-12).
Regardless of his reasons, Paul did wait until he had been bound before he mentioned his Roman citizenship, thereby still gaining the advantage of causing the chief captain to fear (Ac 22:29) and averting another beating.