I was just hired for the summer with a prosecutor's office. I did an intership last break with the PD, and it actually helped me get the job. I live in NJ so I cant speak as to policies in other states, but I think its pretty dumb that a PD or DA wouldnt hire you cause you worked for the other side. If anything that should be an asset not a liability, its important to be able to see things from either sides perspective. As for grades, mine were luckily very good, but the DA who hired me said that grades are not important at all to them, they hire based on personality.