It actually depends upon the requirements of the particular aircraft - I've flown some that require the ignitions to be "ON", and others where it's only necessary to to keep them in "AUTO". The requirement, when required , is to make sure that you keep the "fires" burning in the event of a sudden large power-lever movement - say, for example, a go around. The same thing goes for takeoff, some aircraft require you to use ignitions, other don't. The other times where you'd want to use ignitions is during moderate or greater turbulence and in heavy rain.
[Edited 2003-07-23 23:37:34]