iPhone 3G Battery Charge Time

I have done some research on the charging of the iPhone 3G battery and have learned that it should take approximately 4 hours for the batter to receive a 100% charge (if it is completely dead when plugged in). It's my understanding that 80% of the batter is charged in the first 2 hours and then the last 20% takes an additional 2 hours. Given this is accurate, is there any harm in leaving the iPhone plugged in for longer than it takes to get a 100% charge (as indicated by the plug symbol in the upper right corner of the display)?