Tax and science specialists are much less valuable in Civ3 than in Civ2. Generally you are better off keeping them working on a city hex than switching them to tax or science. So if you have a size six city that needs an aquaduct, you generally are better off just keeping the citizens working on a hex until the aquaduct is built. You might need to change one into an entertainer, but as long as there's a road you should be able to get a total of two or more (food, shields, or trade) from a city hex that is worked as opposed to a specialist that only yields one science or one tax.