I set the carrier frequency specifically for each protocol. Unfortunately, frequency data is even less available than command timing data; the LIRC just defaults to 38 kHz for most protocols, and that seems to work fairly well. But the receivers are expecting a specific carrier frequency, and getting closer to that desired frequency will probably provide better results.

By pushing the frequency all the way up to 60 kHz, I've been able to reduce the effective range to my little Samsung TV to maybe 50 centimeters (or a foot and a half). So I know that it has a range effect for at least some devices.

(I would imagine that bad command timings would have a similar effect.)