Any opening extending from floor to ceiling and exceeding five feet in width should be deducted. The breaking point for the deduction of openings between 50 and 100 SF is a matter of judgment and is relative to the number of openings and their relationship in the overall area being measured. You'll understand why as you read on. Common sense tells us that if we regularly bid a certain type of work, we should notice a relationship between openings and wall surfaces. The size and location of windows are governed by code and design to prevent too much thermal gain or loss, so total window area does not vary dramatically in a particular locale.
Doors are fairly consistent as well.
Some houses might have a storage room, basement, or pantry door. Similar relationships between walls and openings occur in other types of painting work. The most efficient way to estimate labor for specific types of work is to develop a composite rate for masking, surface prep, and painting a surface. Since paint is a fluid applied in thin coats, spreading rates are at best a well-educated approximation. In a very real sense, not deducting average openings is used as a waste factor or cushion, so that enough painting materials are purchased.