Why should you not set your transmit frequency to be exactly at the edge of an amateur band or sub-band?

When you tune your radio to a specific frequency, your transmission does not exist as a single, infinitely narrow line on the spectrum. Modes like voice or digital have 'bandwidth,' meaning modulation sidebands extend above and below your dialed center frequency. If you set your dial exactly at the band edge, your sidebands will illegally spill out of the amateur band. Furthermore, radios are imperfect physical devices. The frequency display might not be perfectly calibrated, or the internal oscillator might drift slightly as the radio heats up during a transmission. Because of sidebands, calibration errors, and thermal drift, you must always leave a generous buffer between your operating frequency and the band edge.