Do I need to use an RNase inhibitor in my RT reaction?
FAQ ID -119

To be on the safe side, we highly recommend the use of RNase inhibitors in all RT reactions, since RNases are nearly everywhere and it is very easy to contaminate samples during reaction setup. The reaction conditions used for RT are well-suited for RNase activity. Even traces of RNase can nick the RNA, causing shortened cDNA products, low yields, and reduced RT-PCR sensitivity.