Baselines data is saved when you save the baseline. If you add tasks after baselining -- there will be no baseline data for the added tasks. If you delete tasks after baselining - the baseline data for that task is deleted, however the summary task baseline does not change.

Again, re-baselining is usually negotiated between the Project Manager and Sponor(s). If you are adding new tasks are they enough to warrant a change order and rebaseline? If you are deleting tasks are you changing project scope and requirements?