Depending on the device You are trying to communicate with, there may be more parameters than the baud rate, number of data bits, type of parity checking and number of stop bits to consider. If I recall correctly, modems use nine lines of the RS-232C interface. Some devices like, for example cash registers, may use hardware handshaking on RTS/CTS lines or on DTR/STR lines.
In general it's good to know how the interface works. You can't communicate if the baud rate doesn't match, but wrong setting of other parameters might kind of work. For example You can easily send data to the device expecting 1 stop bit with 2 stop bits set. Problems start when You try to receive data in such case. You can also use appropriately set parity bit as one of stop bits, etc.
