Leaving the glow starter on will not hurt the engine. Onboard glow drivers are available that allow programming the shutoff (for example, when you move past 1/4 throttle). There is also at least one post (can't recall the site and am not at my usual computer right now) that uses a simple on-off switch and alkaline c cell for an onboard starter that he leaves on for the full flight. This was to solve an issue with 2 stroke inverted motors that can have a problem with oil pooling on the plug causing flameouts at low revs, but both these examples demonstrate that leaving current (at the right voltage, of course) to the plug will not hurt the motor