For some classes such as fighter, you should never hit level 6 before you class.
If you pick a half-ogre necro or mage though, I imagine you will hit level 6 more times than not.
Standard race for a class = easier classing (more around level 5)
Odd race for a class = more challenging and possibly later classing.
At least that is how I view it. I do enjoy the odd race for a class though. The half-orc mages, half-ogre necros and the elf fighters are all challenging to play, yet unique in hier own way.
And gnomes are the bestest race for any class...at least I think so