In Big Bang Theory season 3, Raj start working with Sheldon, or as Sheldon says working for him, and they fight over some desk size in office. However in later episodes it never shown whether Raj is working with Sheldon or not.
Currently I am watching season 6 and still it is not clear they are working together or not.
In season 4, episode 4, Raj moves a desk into the office against Sheldon's will.
In season 5, episode 7, we see the two of them working together in the office. Sheldon puts a snake in Raj's drawer.
In season 5, episode 17, Sheldon moves into his new office, and Raj says, "I’m happy for you, Sheldon. But I have to admit I’m going to miss sharing an office with you."
I cannot see where it has been addressed in season 6 as yet, but one would have to presume it will be if the situation changes.
You can find transcripts of the episodes here.
Sheldon and Raj disagree on how to solve a work-related problem and Raj walks out. Sheldon actually apologizes at the end of the episode and he accepts that Raj was right but doesn't concede himself as being wrong. They agree to continue working together.
Source- Big Bang Theory(Season-3)
Also you will love to have a look at Raj's biography. It describes that Raj definitely need some work to stay in the country. If he would not have been working with Sheldon, he would be deported back to India.
