While any combination of classes that doesn't violate the restricted multiclassing rules or the mechanical constraints provided by Bioware is permitted, the general rule is the more unusual a combination, the more is required to justify it.
In the case of a class combination that appears to be a powerbuild, the PrC applications will come under greater scrutiny. If the character's story and development are such that the development path is perfectly logical, it is far more likely to be approved than if the class requests are clearly purely for mechanical reasons.
As Soulz said, everyone gets a shot.