You probably won't need it, unless you're doing something significantly more complex than the fundamental buy-and-hold index investing strategy.

I'd just use market orders all the time except I'm a little freaked out about momentary (on the order of minutes) market freak-outs as happened a couple months ago. Limit orders priced at the current price (or 1 cent higher) is pretty much just my way of enforcing a sane market order.