It does actually depend on what the money gets spent on. If a person uses it to buy Japanese cartoon porn then the only local stimulation is to the delivery guys and represents a small percentage of the overall cost--assuming you buy your cartoon porn in sufficient bulk. On the other hand if those tax dollars went to lay the first fiber optic lines, then it was a good investment.
Secondly, this idea that private companies are so much more efficient than government really needs to be proved. I've worked in or with 5 Fortune 100 companies and the amount of wasted money and man-hours I've seen boggles the mind. I'm sure government waste is at least as bad, but the difference is when public money is used they are under obligation to give detailed spending reports so you know when it's wasted. Private companies, even publicly traded ones, only have to show a profit and loss sheet. And they do a good job burying those losses as various expenses in order to protect their own asses.
Think of it like open source software. Government, at least in theory, is supposed to be transparent so you can see all the flaws and you are free to try and fix them. Companies tell you to trust them. That they know best and everything is great so you should give them your money (401k). Personally I think they're both good for different things, but if you don't like the government or taxes so much then either get constructive and fix it, or move to a different country.