If you're using a spread, the spread position can often dictate whether a card should be read positively or negatively. For Justice in the position, "What do I fear?" the interpretation would likely be negative. In the position, "What do I need to do?" the interpretation would likely be positive. In a position that describes the situation (especially as the first card in a spread), I would have to see a couple of other cards before I'd know whether Justice has a positive or negative interpretation.
And as Sulis said, context plays a large part. If all the other cards in the reading were negative, it's unlikely (although entirely possible) that Justice would have a positive interpretation.
Instead of reversals, I use Elemental Dignities. With EDs, Justice and the 9 Wands are friendly, so the 9 Wands would reinforce and strengthen the meaning of Justice. How would I interpret Justice though would really depend on the position in the spread as I've indicated above.
HTH,
Rodney