Am I required to sign a bail bond only while trial is in progress?

No, you can be asked to sign a bail bond even after you have been declared not-guilty by the trial court. The court will ask you to sign a bail bond just before the trial gets over. This is because the government can always appeal against the decision. This bond will have sureties and will need you to appear before a higher court if the appeal is filed. This has a 6-month time limit – so if no appeal is filed within that time, you are completely free.