There is no straight answer. Been this way for 35+ years.

Flow control can be electrical (RTS/CTS/DSR/DTR/CD..etc) or logical (xoff/xon). You must accomodate the specific equipment you intend to connect to.

If you intend to host a modem you will find a lot of info including the best intentions of RS-232d. If it is another type of equipment you will have to determine what it needs from it's spec or empiric means.

regards,
p