Do you care about your data getting dropped in the bit bucket? If yes, then use flow control.
Do you have a secondary parity/ECC/flow-control like TCP has? If yes, then you are covered. However, using TCP/OSI as a model, multiple layers do error checking and control to ensure the delivery of data and to keep errors to as few as possible. Let's say your are running TCP/IP, without hardware flow control at 115.2kbps, your effective rate due to hardware flow issues could be terrible.