There was a huge debate over that a few months ago...
I even started a thread to petition for accuracy over anything else in the aliens.
Les (fellow board member, aliens freak and customizer extraordinaire) made some very nice charts comparing the different looks of the aliens over the past movies. I'll see if I can dig them up and post them here for you.
My personal view is this:
Alien: make the figure as close as possible to Giger's sculpture (he actually sculpted the suit on a cast of Bolaji Badejo's body and from that they cast the suit in latex, I think).
Aliens: with a few differences (head, hands and forearms, feet and tail), the basic sculpt should be the same as the original alien, just a little skinnier and taller. There's very good references of the puppets used.
Alien 3: use the rod puppet as template, Period.
A:R: again, ADI sculpted the creatures in small format and from there they designed the suit and the CGI characters. So, just use the original sculpture and the CGI characters as reference.
AvP: Same method... sculpture, CGI. Easy.
AvP-R: Ditto.
I don't necessarily subscribe to the "let's-make-them-men-in-suits" school of thought. After all, the fact that they were men in suits was simply a consequence of technology as a given point in time, and personal preferences. Even now a lot of the aliens action is performed by men in suits, but that doesn't mean that the director's vision is to make them look like that.
Or would you want a figure of a 5'11" guy with black leotards and a couple of pieces of alien "armour" glued on?
Just make the figures look like what the design intended them to look like. I'm not a fan of "artistic interpretation" when it comes to aliens. I'm just **** about that.