Tieflings appear at roughly the same rate among all the humanoid populations, at around 1 per 1000 births. It's a bit higher for elves and slightly lower for dwarves, but not noticeable outside of census counts. Everyone accepts that they are a fact of life, but nobody's particularly happy about it. In most cases, fear of tieflings hangs in a delicate balance with fear of angering one, and they are treated with the bare minimum of politeness. The Orcs believe that tiefling children are sent by the gods to give their families magical powers, and it's considered an honor to have one in the family, but they're still treated with distrust and unease. (They are also killed if they don't manifest any magical powers, but their value is worth waiting for, so many are able to escape to the cities if they realize they won't be able to fulfill their “destiny” before their families do.)