Why AFSK data modes feed audio tones into the SSB transmitter, and the linearity of that chain determines the spectral purity of the signal. Automatic Level Control works by reducing gain when drive is excessive, which compresses the waveform in a nonlinear way and generates intermodulation products and spurious sidebands. The result is a distorted, wider-than-necessary signal that splatters onto adjacent frequencies and decodes poorly. The correct practice is to lower the audio drive until the ALC meter shows little or no action, then use the transmitter's own power control to set output.
Watch out The overheating idea is a distractor: ALC actually reduces drive, so it does not raise dissipation, and heat is not the concern with data modes so much as duty cycle. ALC also does not invert modulation; it only changes gain.
Digital modes: turn audio down until the ALC needle barely moves. ALC action equals distortion and splatter.