It depends on the specific crankset. Some two-piece cranksets are designed to be installed without washers, while others use two or more. Truvativ/SRAM publishes the chainline dimensions of the cranksets they sell in the "tech docs" section of their website. You may be well rewarded by searching Shimano's tech docs for information as well.

As long as the manufacturer recommends a configuration that allows you to run the cranks directly next to the bearings without a washer, and the resulting BB width wind up the same, I don't see any reason why you can't shuffle those washers around to adjust your chainline to your heart's content. Subtracting washers is a definite no-no. Adding washers will reduce the amount of left crank to axle engagement, and you do so at your own risk. You'll probably want to measure to make sure the crankset you buy will have enough chainstay clearance though.