Wing nut
As a rule you should be using a headset for every departure/pushback or engine start. The only time you would not use a headset is during thunderstorms. You will find that most Captains will refuse to push and start without one with the exception during thunderstorms.
If you have not been trained for headsets mate you should not do them at all. If you had an engine fire it is important to pass concise details to the skipper as he wont be able to see the engines more so on the CRJ.
Cover your back and get the correct training.
Rgds
Chef