I assume your talking about powered monitors. Shouldn't hurt 'em and could actually be good for some of the internal components. I would make sure to turn the inputs down though when not in use to keep 'em from getting blown out by any electrical/signal spikes.
i heard that leaving your computer on is better for it as it reduces condensation on the hard drive (less temp variation = less condensation). but i would thing leaving powered monitors on would have some adverse effect.