At most of the agencies where I have worked, the agency hired indiscriminately and sent 'whoever' to staff the cases. The only difference would be in the rate of pay for the RN versus the LPN. On occasion, there would be a situation where an RN was called for, and they would go out of their way to staff that particular case with an RN. I have never been told to my face that they hired one over the other, even if that was the case. I've always seen more LPNs on the job, primarily, I think, because RNs can get better jobs at higher rates of pay, generally speaking.