perhaps this has been stated, as most folks who find the need to intensify do so because of thin negatives. Selenium, which is used for this purpose will effect the whole negative not just the shadow. Rule of thumb is that it raises the Zones one step up. Many years ago it was very common to use selenium on negatives all the time to ensure archival properties. when doing so the negatives had to be underexposed a bit to allow for the increase in density.

This goes along with reducing, a "only when necessary', last resort technique.