Do you really always need antibiotics after root canal treatment? Can you just wait and take it when you see infection started only.

Yes. The root canal is used to treat an infected tooth that is full of bacteria. Antibiotics will help eradicate the infection or bacteria. Why wait and see if an infection may arise. Simply take a round of antibiotics to ensure that an infection does not occur or is eradicated.
No. Antibiotics are not required after all root canal treatment. Various medical conditions that effect a patients response to endodontic treatment guide the use of antibiotics as well as the presence of a systemic infection. It is always best for you to understand the purpose of any medication before taking it. Discus this with you dentist so that you will be more comfortable taking the medication.
Depends. Depends onthe presence / absence of infection at the time of treatment. If infected, most endodontists make a judgement call regarding use of antibiotics. Most prescribe them to give you best chance of uncomplicated healing. If you wait, the infection has already gotten the upper hand and you are trying to catch up. You already did this once, why wait and do it again ?