If you screw your die all the way down so that it contacts the shell holder before tightening the locking ring on the die, you will be squared up. You will also ensure that you are full length sizing your brass, as the stroke forces the shell all the way into the die until it is stopped by the shell holder. If you put a washer between the shell holder and the die, you won't be full length sizing the brass.

The "squareness" problems arise when you try to use a full length sizing die to neck size your bottle necked rifle cases. This can be done quite satisfactorily if you back the die away from the shell holder a bit so that it does not contact the shell holder on the stroke. However, you DEFINITELY need to square the die up first before locking it in place. This is where the washer or shim comes into play.

I personally use a Starret thickness guage to do this rather than a wsher, because I share the other guys' concerns about the flatness of an inexpensive washer. I use this guage because I happened to own one; however, these are rather expensive, and you can use a piece of shim stock and get the same results. I have gotten satisfactory results by leaving my shell holder at the top of the stroke, then screwing the die in with the guage betwen the shell holder and the die, until the die is tight against the guage or shim. The reason for doing this is that the outer threads on the die are coarse enough that you may not be exactly square with the shell holder; there is a fair amount of "play" in these threads. I then tighten the locking ring on the die, lower the shell holder and remove the thickness guage. This sizes the neck perfectly, does not contact the shoulder, and provided I use care in centering the expander ball each time, I get good concentricity, measuring about 0.001" runout.