swh,
I got my instrument licence 20 years ago when, although LRG IRU's were about, the mainstay were the Litton and Carousel INU's which used standard rate gyros (the Carousel INU platform was actually mounted on a rotating "carousel" to reduce any steady state errors. If you opened it up to look inside it was a joy to behold!).
I digress, When I went on courses we were told of Schuler effect and informed that it was accounted for in the system. Hence my reference to unit removal if a 80-90 minute drift oscillation occured.
I don't deny that INU's may exist which can have their Schuler circuits disabled so that the Schuler period and amplitude can be determined (that is why I clarified my comment with "AIRCRAFT INU") I just have no experience of them.