There are two more FM which you should use OPM_DM_PRECALC_CLEAR and OPM_DM_PRECALC_ENDCLEAR (This is similar to what you do In refresh data source in UI)
It is a best practice when you change something in query or data model to refresh data source or execute OPM_DM_PRECALC_CLEAR & OPM_DM_PRECALC_ENDCLEAR.
and OPM_DM_PRECALC_DELETE should be executed when you have loaded new data.
I agree to john that this must be done.As the report xmls are cached and saved in these precalc tables.
This is done to improve the performance of report running.
So when you have new data in reports you should delete the cache and rerun the report to add the tables with new cache.
Regards,
bidisha