3.3V TTL to other formats and impedance matching
If you have a long transmission line at the 3.3V signal, you only have to worry about terminating that properly, however if the output has 50 ohm source termination, whatever reflection happens at the end will get terminated at the source, so it may not be a problem. Then, the RS232 driver outputs have slew rate limiting at the driver output and it requires no termination, as serial port lines do not use impedance matching, they use voltage as the signal so output impedance is low and input impedance is high.