Campag (ISO) and Shimano (JIS) tapers are different. They are both 2 degree tapers that start abut 12mm wide. But ISO ones are .2mm narrower at the start (IIRC) so a JIS crank on an ISO taper will go further in than it should and and an ISO crank on a JIS taper will not go as far as it should. Whether this will make a difference will depend on the crank shoulder and whether there's not enough or too much taper in the hole. The chainline will be different too if this matters.
Best to use the right taper if you can, if not it may work fine - or not.