I normally 100% of the time keep the constant 'scruff' look going but of course shaved that for my interviews. I would play it safe and keep it clean, but that's just my opinion. Then again, it doesn't take me more than two days to re-grow the scruff. A lot of people (myself included) additionally stressed about their clothing choice and after going through this process I will say that it probably doesn't matter as long as it looks clean and professional. I remember reading people having a fit over black vs. blue vs. grey suits, what color socks to match with them, what color tie, etc. and in the grand scheme of things it probably doesn't matter as long as it's professional (don't go in there looking like a pimp with the pink suit and chains). Do what makes you feel comfortable and will allow you to most be yourself at the interview since they will be paying a lot of attention to your words.