We always did this in my school, and then on the day of care the student was often more knowledgable about the patient than the nurse! The student was able to take the time to read the H&P, Social Worker notes, and all of the other little things that an RN would not have time to read before that first physical assessment. It actually makes my school well-regarded in hospitals because we come so well-prepared.
The only downfall was that you might pick out a patient, write up their meds, patho, NANDA and interventions, and then go in the next day to find out that they had been discharged.
So you picked up another patient, starting from scratch, and all of your work was for nothing. The way to get around that, and to answer your question, was to wait until after the Dr had done rounds and made decisions about how long the patient was staying/what care the patient was to receive, and then talk to the charge nurse about it. For us it ended up that 1300 was about the earliest you could go in.