Smoking on an empty stomach gets you more fucked more quickly, just like drinking alcohol on an empty stomach. I wouldn't recommend it though, it could make you sick. It all depends on personal preference at the end of the day, and if your weed is running low or is expensive to buy then rather get ratted on an empty stomach.

P.S. I heard on another thread that wetting your bud gives you more smoke in a bong - is this true?