Arguments are only interesting in the context of their keyword. Therefore, put zero or one argument messages on the same lines as the receiver. Smalltalk distinguishes between code which is executed immediately upon the activation of a method and code whose execution is defered. To read code accurately, you must be able to quickly distinguish which code in a method falls into which category. Code should occupy as few lines as possible, consistent with readability.
Short methods are easier to assimilate quickly and they fit more easily into a browser.
On the other hand, making it easy for the eye to pick out blocks is a reasonable use of extra lines. One more resource we can bring to bear on this problem is the tendency of the eye to distinguish and interpolate vertical and horizontal lines. The square brackets used to signify blocks lead the eye to create the illusion of a whole rectangle even though one isn't there. Therefore, make blocks rectangular. Use the square brackets as the upper left and bottom right corners of the rectangle.