I am wondering whether I can simply use the harmonized occupational categories in the variable occ1990 without applying any additional weights. For example, in cases where an occupational category from the post-2002 occupational classification system draws from multiple 1990 occupations I believe the census recommends including the person in the various 1990 occupational categories and weighing each case according to the contribution of each 1990 occupation makes to the post-2002 occupation. I am concerned that the occ1990 appears to imply a one-to-one correspondence between occ and occ1990 for every year.
You should not need to perform any additional weighting procedure due to the modifications performed when creating the OCC1990 variable. The harmonization process used to create OCC1990 simply assigns values of OCC as if the occupation coding scheme used in 1990 persisted in additional years. That being said, if you want to calculate representative statistics for any given year using the OCC1990 variable, then you should apply the appropriate sampling weights as normally recommended.