There was a guy who got caught selling a lot of marijuana and didn't get in much trouble because he was paying taxes on it since day 1.
http://marijuanastamps.com/state-stamps.htm
Mostly, it's another way for pigs to steal from people. The idea being they can not only throw you in prison for dealing, but then use tax forfeiture laws to take everything you own.
The best you could hope for is some sympathy from the judge and/or not also getting slapped with tax evasion.
Source?