The dimensions on almost all drawings for machined components are assumed to be after plating. However, plating thicknesses generally only matter when machining parts with tight tolerances (<0.001″).
Most plating processes only add a couple tenths or less (<0.0002″) of thickness per side. This includes chromate conversion, Type II anodizing, and electroless nickel plating. Measuring parts only before plating is generally acceptable in these cases because the part tolerances are typically an order of magnitude higher than the plating thickness (thousandths instead of tenths of an inch).
If there are any part tolerance requirements in the tenths range, the plating thickness must be accounted for when measuring the parts before plating. Some tight tolerance features, such as reamed holes, can be masked/plugged before plating to control the diameter.
Unlike most other plating types, hard coat anodize (Type III) adds 0.5 to 1.5 thousandths of an inch (0.0005″ – 0.0015) per side, so the thickness of the plating is significant for many parts.
Paint and powder coating finishes are generally much thicker (several thousandths of an inch) than plating and can vary widely between batches of parts. Unlike plated parts, it is generally assumed that the part dimensions shown on drawings apply before painting or powder coating. Drawings will generally specify masking requirements for particular holes or features in which paint buildup may cause fit issues.
In summary, plating thicknesses typically do not matter when it comes to part tolerances in the thousandths range, but they matter for part tolerances in the tenths range. In practical terms, parts with standard tolerances can be measured before plating. Parts with tight tolerances must be measured before and after plating.