Who can say? But most of the finished designs take a lot of their leads from Giger, whether it be early pre-production stuff on ALIEN or Dune, so even if Giger only did a few sketches that Ridley found 'interesting' I'd be surprised if that didn't act as a springboard to the other artists. As far as I can see Gigers sketches are the earliest to feature a pointy head, a lot of the stuff by others in the early design stages are more faithful to the original alien.
Despite all the creatures having a softer more organic feel there's a healthy respect for Giger in the finished creatures, and the Engineer is practically Gigers' generic 'human' victim.