they are absolutely necessary to loosen the cones. crescent wrences aren't thin enough to turn the cones and you may wind up just loosening the locknuts which hold the cones in place.

On the plus side, since cone wrenches have to be super thin, they are usually stamped from sheet metal and are pretty cheap to buy.

Having it done would probably be pretty reasonable too, though.

Get on parktool.com or sheldonbrown.com or hit your local library. Make sure you know which is the cone and which is the locknut.