I don't think it is. Taxation may share some qualities with theft, but when contained to it's legitimate purpose - to fund government - it's hard to see it in the same category is arbitrary stealing. When it steps outside that purpose, and becomes a general tool to bully and manipulate society, it's worse than theft.
I don't know that I would say theft holds any "qualities". What i will say, is that this sounds a lot like an argument I heard from one of our last decent congressman.
"I don't tell my patients when they come in that they have a touch of pregnancy. You're either pregnant, or you're not."
Same applies here, it's either theft or it is not. in the case of compulsory taxation, it is theft by the use of force and violence. The reason the state is not legitimate in my eyes is because it monopolizes the use of something it otherwise attempts to condemn and mitigate throughout the rest of society - force and violence. If you build a society on such blatant hypocracy as is done in comtemporary Statism, the results will ultimately be the same. Which is like I said, the power to tax is, was and will always, be abused. Always.