This question mainly goes out to the writers, but other readers should feel free to answer too. I've been wondering for quite a while now, which versions of the aliens the writers or readers have in mind when they write or read stuff involving aliens like the Tellerites or the Andorians, who have gone through numerous real universe make up changes? Personally I usually tend to go with which ever version I like best, which is usually the newest and most realistic one. For example the Enterprise versions of the Andorians and Tellerites, and of course if there is an in universe reason for the different look, like with the Klingons then I just go with whichever version matches up to that time period or place.
I tend to imagine Westmore's ENT-style Andorians and Tellarites no matter what era the story's set in.
I usually imagine TOS-style Klingons.
When I can, I try to imagine TOS-style Romulans. Sometimes I can't if the character is specifically described as having TNG-style forehead ridges, though.
I tend to imagine ST:FC-style Borg -- except that now Greater Than the Sum
has established that TNG-style Borg were actually androgynous drones grown by the Collective, so now I see no reason to mentally retcon them if they're Borg from the 2360s or earlier.
I tend to imagine DS9-style Trill and to pretend that the other style doesn't exist.
I try to pretend that Vulcan females don't all have Moe Howard haircuts.