An acid wash is the stripping of a thin layer of plaster, exposing fresh plaster beneath. Therefore, it is not recommended to do it annually, which will accelerate the need for re-plastering. Most plaster coats are in excess of 1/2", so a careful acid wash every 5 years won't hurt.
A general rule of thumb for determining if your pool needs an acid wash: If you can see the bottom of the shallow end floor, you can probably bring it back with chemicals, cleaning, and filtering. If the floor is not visible, the cost of the chemicals and labor may exceed the acid wash charge, and take much longer. Also, extensive algae blooms or neglect will stain plastered pools, making an acid wash desirable.