Depends on the setting really. I tend to think of tieflings as rare, because the implications of them being not-rare are a little too profound for a "normal/generic/kitchen-sink" setting. I've always thought the idea that they breed true to be interesting though, as tieflings would essentially assimilate other races. Male tieflings would always impregnate females with tiefling babies, female tieflings would always birth tiefling babies. I thought about applying the same thing to dragonborn, would be an interesting play on both their magical heritage.
Anyway, it really depends on the setting. How established are tieflings in the setting? Are the random and rare? Do they have empires? Are they true-breeding?
I don't know what you qualify as "normal". Tolkein? Forgotten Realms? Greyhawk? Something else?
As above, I tend to consider tieflings "normal" but rare.
BUT, I also like to let players define their own tieflings. If you want to play a more tropey "sexy girl who looks like a succubus but isn't" okay. If you want to play the ram-headed, thick-tailed style, okay. If you want to take inspiration from other demonic races, such as Pathfinder's Blood of Fiends that's fine too. Heck, if you want to look totally human I'm okay with that too. For my "normal" games, your race doesn't factor in much unless the group has an expressed interest in that.
