when i have used them in the past for extra cash or what not. i always tell them the lowest pay i will accept, the days/shifts i am willing to work and even the type of work i would be willing to do.
if i go to a place that i don't like, then i tell them that i won't go back there again and to find me somewhere else to goto.
remember the staffing agencies make their money off of your labor. and also remember there are butt-loads of staffing agencies out there, so go to a few of them and see who is going to treat you the best. also sometimes if there is a job drought, being with more than one agency could make the difference between working a couple days a week or working everyday.