I stand corrected, Tunalover. I have mostly worked with SI units during my (admittedly short) career, and I've never done an imperial drawing under ASME Y14.5M-1994. However, unless my understanding is incorrect, that "drop the leading zero" portion of the standard only applies to units of inches and only when displayed on a drawing. I'm not sure that qualifies for as sweeping a statement as "common US practice". This next bit is pure conjecture on my part, but I would guess that the reason for dropping the leading zero for inch dimensions less that 1 is simply to save space and prevent crowding on tolerances, etc. Since millimeters are so much smaller than inches the leading zero can be preserved without sacrificing paper space or precision.
I do always use the leading zero, even in hand calculations because it adds clarity and makes it harder to accidentally miss the decimal point, especially if I think someone else may need to read it later. What are the benefits of dropping that leading zero?