YES! you must turn the power off to the circuit you are working on. Electricity does not play, you make one mistake and it will kill you. I have been an electrician for 20 years and still turn the breaker off every time.
We all can do drywall, or frame a porch up or paint a car. If they don't turn out like the pros did it, no big deal, the wall is rough and uneven, the porch slopes and creaks, the paint job looks good only in the dark and while going by at 55, we live and learn. You work with electricity and it doesn't turn out like the pros do it and you end up dead.