I leave my computerized machine on for the duration of time that I have to spend on a given project. I may be ironing; answering phones; replying to emails during that time as well as sewing. When I know I'm done for the day/evening then I'll turn off the machine. Otherwise it's on for (relatively) short inactive periods. Like most electronics, it takes more power to turn on/off the given machine than the power you are saving by turning it off.
I even (gasp!) leave my computerized machine plugged in to it's surge protector when there is an electrical storm! Knock wood...we've never had an issue with lightening affecting any equipment in our home in the 27+ years I've lived here, nor has DH in the years prior to that.