obd test on my VW polo .1.4, 2004 mdl?

Hi... i am back after having seen my car OBD tested. the Mechanic says the problem is ignition coil.. I had changed one just as the "CHECK ENGINE LIGHT" poped up. I bought one ignition coil for #80.it is original. Now MR. Mechanic advises me, if I change one col I should change all the rest. Even if they are okay. Sounds crazy for me.... what do you say