3 months from the date of bringing your vehicle in Bangalore is safe period . Legally no one can touch you if you have other proper documents with you. Just keep your vehicles transportation bill with you .
I will tell you, What you need to do to pay road tax .
1. NOC for migration of vehicle from one state to Karnataka, The State RTO where you registered your vehicle will give you the NOC .
2. Registration copy .
3. Your address proof in Bangalore (Rent agreement , HR letter , etc )
4. Insurance copy of your vehicle (must be valid).
Now find a bangalore one center or RTO office .
Now one important thing , you asked to register your vehicle in bangalore । Insist to pay only road tax. Paying road tax is enough to drive on bangalore roads. No need to pay bribes or go to agents . Specially the agents who roams around the B1 and RTO centers . If you go to some one who has permanent office then it is ok . You may need his office address to lodge a FIR :)