Unless YOU are willing to cover warranty issues of users running
beyond the mfg's recommended voltage, it's not wise to make such
a wide ranging blanket statement.

SOME ignitions can handle higher voltages -SOME- not all.

Saito, Evolution, 3W are a few examples of those that state they can.

DL;
"The DL50 ignition system is designed for input voltage ranging from 4.8 to 6.0 volts, no more and no less."

DLE; same as DL above

If some t/s folks said different, I'd be inclined to stick with the manual they print until those get updated.
There's NO advantage in running higher voltage to these ignitions. The spark isn't getting any bigger.

Voltage range of 4.8 - to 6.0 is what most of these ignitions require and is clearly stated in the owners manual.
No more, no less.