Count A Strong, they will know what the firms they place people with want/expect and IMO will want to see how you will present yourself to an employer. Their ability to place people will depend a great deal on the quality of people who they send...firms wont go to agencies who send them people they can't/won't employ. I still say if you are going for a white collar job or something in hospitality then dress smartly. If its something in the creative sector then perhaps a bit more leeway to be more individual in your dress.
lets face it, today's job market is not that great, why would you do anything to spoil your chances?