A lot of times even I don't know my character's full backstory, so saying he grew up in Waterdeep instead of Icewind Dale isn't a problem. The higher level the character, the easier it is to say something happened earlier, but it's not hard anyways. If you're an Evoker going into Elemental Savant, it's perfectly reasonable to say that the reason you're so interested in flames is because you had Salamanders in your fireplace as a child. Oftentimes, the RP requirements for a PrC would be impossible in certain campaigns, and ridiculously easy in others.