Let it run all the time.

I agree. I run mine all the time. I suppose it wouldn't hurt to cycle it on an off for a brief period but I prefer the steady movement of air through the seedlings.

Dave

Not to hijack, but I'm running a 400W metal halide. Should I have the fan positioned at the level of the lamp or the seedlings? I'm planning on starting the MH 2' above the seedlings.

At the level of the seedlings. The fan will simulate wind, and the "wind" will produce hardier and stockier plants.

Your HID lamp radiates light and heat much like the sun. For your seedlings they are benefiting mostly from the light and re-radiating into the air the heat that they have absorbed along with the light. The air movement from the fan provides a push against the growing seedling causing as a response a stiffening of the cells in the seedling. Many university tests have been done to show that a tree grown continually tightly staked will never develop a proper trunk to support the tree. Think of your seedling as a miniature tree! Al

Thanks for the tips!