What type of agency are you considering? An employment agency that finds you a job? ... or ... An agency that hires nurses and then sends them to employers for temporary assignments?
As a new grad, you may not be qualified for jobs at the second type of agency. They may only hire nurses who have some experience and who are prepared to walk into a health care facility and be competent right away -- without a lengthy orientation.
If you are talking about an employment agency that would help you find a job at a local hospital, there is probably no need to go through such an agency unless there is something uniquely challenging about your situation. They types of jobs that you would be qualified for are entry-level staff nurse positions. They are standard jobs and come with a standard set of expectations and compensation packages.
What you should be looking for in your first job as a nurse is a strong orientation program and an organizational culture that is supportive and nurturing -- in a speicalty that interests you. Those factors should be far more important than the compensation and more important than the work hours, etc. After you have progressed in your development to the point at which you are a competent professional nurse with some experience under your belt, then you can focus on some of the other factors in a job search