Since the back-story of tieflings make their infernal characteristics a perversion of their baseline humanity that is inflicted upon them as a reminder of the pact, I think that treating the expression of those traits as if they were some narrow variation of evolutionary development is unnecessarily restrictive. The traits can and should have a certain randomness about them. I believe that the best way to inject unpredictability into physical traits of devilish origin is to allow each player to choose their own.
Remember, DMs, that tieflings and dragonborn are baselined as uncommon. Tieflings avoid gathering into concentrated groups to prevent other races from seeing them as a threat. One tiefling, like one biker, will be noticed, but other people don't flee at the sight of one or organize a mob of pitchfork-wielding townsfolk to chase them off. But imagine yourself pulling into a road-side eatery for a bite, when a hoard of thirty to forty bikers pull up in a swarm, sporting colors and parking in precise rows of gleaming crome and growling power. There are few among us whose survival instincts wouldn't kick it up a notch.
YMMV, and that is the beauty of Dungeons and Dragons, but assuming that you adopt the baseline, tieflings will add color and interest to the makeup of your world without overwhelming it. I don't see any reason to prevent the players from realizing their internal conceptualization of what their tiefling looks like to support their idea of the character overall. Certainly, the most exaggerated cases will result in a great deal of sideways looks and subtle (or not-so-subtle) prejudice, but that is the essence of the joy of playing a non-human character. Keep in mind that your players control only the appearance of their characters. You control the rest.