If your EMI concern is strictly "radio", amplitude-not-to-exceed
within any given slot, spreading can help you "game the system".
But every individual pulse will emit the same impulse energy, to
be received by neighbors, regardless of how the rate wanders.
You would have to understand your sensitivities to determine
whether what might be displayed on a spectrum analyzer, is
really the interesting part of the story. Just like transient analysis
and AC small signal analysis can show you the same thing if you
are looking at gain/phase with a small sinusoidal stimulus, but
disagree quite a bit when things are not so ideal.
In my view, spread spectrum clocking is a way to pass standards
tests, but not a cure for envelope-of-noise or spike type issues.