When giving concurrent chemoradiation therapy, is it important that the infusion be prior to RT as opposed to after RT?

Is there data supporting the idea that chemotherapy must be onboard prior to delivering radiation for maximal radiosensitization (particularly for head and neck or cervix)? Do you ever start radiation +/- 24-48 hrs from C1D1 for chemotherapy due to logistical barriers? Do you prefer chemotherapy to start at beginning of the week vs. end of the week?