This isn't the case.

JIS stands for Japan Industrial Standard, and is comparable to DIN (Germany) Norm Francais (France), BSC (England), Ansi (USA), and later on ISO - International Standards Organization which seeks to streamline things with a single worldwide standard.

The ISO headset standard is the old Italian standard probably because of Campagnolo's success in establishing it worldwide.

However whatever standard bikes are built around, the methodology is the same. Bikes intended for the Japanese home market were machined to JIS the same way others were machined to ISO or other standards. The machining after assembly was done on all better brazed bikes because these were built with excess material, which allowed for the machining to correct for distortion during the brazing process.

Some low end bikes skipped the machining operation by using tubing sized to spec (or close enough) and simply forced a headset into the ovalized tube.

Whether a frame needs to be prepped (or can be) isn't a function of the standard it was built to, but the process and thinking that went into the construction.