We all know that tiefling and aasimar are a shade underpowered for +1 level adjustment. Sean K Reynolds has admitted this in the past, pointing out that everyone would play them if they were +0 level adjustment but some people would still play them at +1, and that a better solution than the level adjustment system couldn’t be found.
If you want to encourage the planetouched races in your game by giving them a little boost, here’s my house rules on the topic.
First, make the race a template – why should only humans get to have extraplanar ancestors? Thus there now exist in your world aasimar elves and tiefling dwarves. The character gains both the benefits of the base creature as well as the planetouched race, and these stack – for example, a tiefling elf has (amongst other things) +4 Dexterity, -2 Constitution, +2 Intelligence, and -2 Charisma, and treats both rogue and wizard as his favoured class. He retains the Outsider (native) type regardless of the base creature.
A “plain” aasimar or tiefling in this variant is simply a planetouched human. He is as the standard tiefling but with the bonus feat, the bonus skill point per level and can treat any class as his favoured class. An alternative to this is that
The effect of this on your game world is that planetouched should be more common to fit with how often players choose them, and there should be some explanation for this such as planar war in the world’s history. This also means that you will have planetouched monsters – good-aligned aasimar minotaurs guarding holy sites, well-organized bands of tiefling orcs with devil ancestors – but don’t overuse this! Remember that planetouched races have resistance 5 to several elements, which at low levels can make opponents immune to most of the wizard’s offensive spells.
In truth I haven’t worked out yet how tiefling-as-a-template affects a creature’s challenge rating – perhaps it brings a CR1/2 creature up to CR1, but then perhaps it’s not even worth that much. It’s probably something best decided on a case-by-case basis.