Depends, I suppose.
One could make the argument that since "pulling" is inherently more efficient than "pushing" (think of our front-wheel drive vehicles), if you have an engine with only one powered truck, putting that truck forward would be "best" regardless of hood length. For example, Gilbert's Flyer GP7's powered truck is at the short hood end:
However, most scale models have a centrally-located motor with both trucks powered, so it becomes a matter of taste: (O scale Red Caboose and Atlas N scale).
Personally, I like my EMDs to run short hood first; Alcos are fine either way: (Weaver RS3).
One of the nice things about Euro engines is that (for the most part) they are the same at either end, so there is no "front" or "back" end! 
(O scale Rivarossi and HO scale Roco DB "215"...)
One last picture: this HO scale Roco "V200" is probably the single best running engine I've ever owned, in any scale. Super smooth, quiet and heavy so it can pull as many cars as you'd want. 
Mark in Oregon
