How will the results of my background check affect my job possibilities with a Big 4 accounting firm?

I have two possession of cannabis charges from over 5 yrs ago. I never got convicted of either charges and they are in the process of getting expunged. However, I recently got arrested for a DUI and I am 100% sure I will be dropped of the charges after I go to court. How does this look to employers and at what point during the interview should I bring this issue up and explain?