When you are warming up by pairs, take a look at the height of each blade above the water (it is useful to look at hand heights as well, but this is not so easy from the cox seat). At square blades the blade should pass a 1 or 2 inches above the water surface, and on the draw should be pulled through 1 or 2 inches below the water surface.
If you see that one of the blades is very high above the water every stroke and the rower’s hands are not too low on the recovery, one solution is to get that person to remove a washer. This lowers the point at which the blade is pivoted, and so in order to pull through at the same height, the rower will carry his blade closer to the water. The same effect would be achieved by making the rower sit on a seat pad.
On the other hand, if a person is having difficulty getting their blade across the water on the recovery (especially at square blades) – i.e. if the blade bottom ticks or scrapes the water on the recovery or worse and the rower has difficulty getting his hands over his legs – then that rower probably needs a washer added.
This all assumes that a rower has a proper “natural” height at which they draw the oar handle toward them, or carry it away from them. By adjusting washers, you are allowing them to maintain this “natural” height and are adjusting the boat to fit the rower.
Novice rowers will not have a “natural” height; they must learn this. This is why extended periods of square blade rowing are useful — it teaches the necessary height differences between the drive and the recovery.
As the differences are learned, seat cushions (risers) and washers can be adjusted so that, on the drive, the rower can draw the oar through with the hands pulling to just below the sternum without the blade washing out.
Taller folks will pull below this mark due to their longer torsos. Shorter folks may have difficulty clearing their thighs if they are adjusted to this height, and may need to pull in higher to maintain clearance when rowing square blades on the recovery.
In general, the fewer washers under the pin, the more stable the boat. If the boat feels floppy, removing washers will generally eliminate the floppiness.
Different boats will need different adjustments; heavier boatings of rowers will generally require more washers underneath the pin. Lighter boatings of rowers will generally require fewer washers.