Depends on many factors.
1. First and foremost is the crews ability level. For novice crews its all about timing up front, and moving the worst technical/timing paddlers as far back as you can get away with. Even for intermediate crews - figure out who puts their head down during the race and mucks up the boat and get them as far back as possible. It's all about damage control!
For advanced boats the opposite can be true - get paddlers with good feel in the back where they are able to find resistance in fast moving water. Ideally guys or gals with lots of experience paddling but might not be super impressive in the gym usually work out great back there. You want paddlers with lots of raw strength as far up the boat as size reasonably permits where the water is nice and "heavy". Up front in the strokes you want someone with experience, the right rhythem, who the crew respects, and can execute your race plan while being cool and collected. The worst person to have up their is a type A who can't control themselves during a race - rather someone who responds to the drummer but in a controlled manner.
You might also want to "thread the boat" during practice - for crew development, but maybe not your final lineup for race day. Try to sandwhich people that need to improve between those who are doing what you like. Boat order can make a big difference for the speed in which a paddler and by extension crew develops - then finalize the boat order as you approach your race.
I also like to integrate men and women instead of all women up front, or in back, or a combination of the two. It seems to motivate paddlers better as the women try to out paddle the men and vice versa. Seems silly but it works and helps to get rid of "hiding" in the boat where people aren't putting in their full effort.
2. Wind direction - headwind = weight up front, tailwind = weight in the back. Otherwise I agree with the posted who said slight forward lean when not paddling. A well balanced boat should feel like it is plaining (i.e. riding up on a wave) when paddling in deep water.
3. Size- Tallest in 6, biggest in the middle, make sure you have strong paddlers in 3-4, flexibility up front, narrow bums in the back,