Impressions are a delicate balance between the experience of the audiologist, the materials/methods they use to take impressions, and how well they adhere to the manufacturer's instructions, which is paramount to getting impressions that fit well.
Just because someone was trained by Sensaphonics' Golden Circle (two-day seminar only) doesn't mean they will know exactly what to do with all manner of impressions, including acrylic ear molds, nor does it guarantee that they'll necessarily keep up with current literature in audiology. Silcone CIEMs do not require open-jaw impression techniques, either, which is vastly different from impressions for acrylic molds.
Most acrylic CIEM companies will assume an open-jaw, bite block assisted impression taking technique, along with medium to high viscosity material --- a position championed by both Westone and Starkey. The material used affects curing time, as well as compliance to the ear canal and the shape of the pinna.
Figure 1. 
Figure 1 shows that open-jaw impressions with high viscosity material produces the least variances in results at all three critical areas of impression taking within the canal.
From Piranski & Berge, "an impression for a higher gain instrument should be taken with a more viscous silicone and the patient's mouth wide open. This will maximize the instrument's acoustic seal and lower the risk of acoustic feedback. To ensure the instrument's comfortable fit deep in the ear, the canal tip on the impression must be skillfully trimmed. The ear tissue at and past the canal second bend is more sensitive to touch than the cartilage between the bends and will not tolerate pressure from the instrument. The Au.D. students who focused on hearing aid remakes secondary to; acoustic feedback, insecure fit, discomfort, or poor sound quality, found that for 114 out of 136 (84%) patients, taking open jaw impressions with more viscous silicone remedied or diminished the fitting problems. Of course, had such impressions been taken in the first place, the majority of the remakes could have been avoided. Unfortunately, some earmold labs are against open jaw impressions and more viscous silicones because they lack confidence in their skills in processing such impressions into comfortable hearing aids." 
In terms of the pinna, Heir Audio wants the superior junction of the tragus covered and compressed as well , presumably to ensure that the soft cartilage of the tragus is sufficiently distended for a good seal.
Not really, I NEED to know where the socket should be placed, and it amazes me how many impressions we get without that area covered!
I could rant but why bother....
By the way, we do not trim, sand down etc the ear canal portion of the ear mold impressions, as we try our best to keep everything as intact as possible. Truth is, Hearing Aids are far more forgiving than CIEMs.