When do you do it? How much do you take off? Is it a good idea to take off enough for a clone? And what about yeild?
do it between 4th and 5th node if possible, just cut off the little leaf bud right at the top, the only way you could 'clone' a topped head would be if you use tissue culture, which is a pretty advanced way of rooting small cutting, from what i can gather.. yeild it can affect, but if you give it enough time to grow back enough you should be fine...

just my two cents...