I am connecting the RF102S to a device that does not look at the RTS line, but will always set CTS high when it is ready to receive, regardless of whether the RF102S is requesting a send with RTS. Is there a problem leaving RTS open?

RTS is an output from the RF102S and will usually be high, it should be OK if your external device does not use it.

CTS is an input to the RF102S, it will monitor this signal when sending out data, if it is high, it will send, if it is low, it will stop sending.

RF102S toggles DTR when it is trying to initialize and hangup the external modem.

RF102S monitors DCD to see if it is online or offline.