I believe that if voluntary tax would not replace the obligatory taxes, then it could work out well. This is because it would not change the amount of revenue the government would receive and could only increase the total revenue they earn. If people are generous enough to donate more of their money to help their nation, then I see no problem with that.
This would however be promoting greediness and give humans the idea of working for themselves and not sharing. The more a person sees the art of giving back in which ever way its done, then they will not mind doing it voluntarily, we should work together not against each other.