IANAL but example taxable transactions might be to sell bitcoins for a gain, or to use appreciated bitcoins in a barter transaction. Losses are also taxable transactions, but in a good way.
Mining is awkward, since I believe mined coins are taxed as income on the day they are mined, even if you don't transact.
Best to talk to an accountant, of course, and not rely on the advice of a random HN commenter like me.
Neither because buying bitcoins doesn't help you not pay taxes.
If you have legitimate money that's reported to the IRS and taxed, what you do with it is largely irrelevant.
If you have illegitimate money (selling drugs, working under the table, etc) then you're not paying taxes on it to begin with. Buying bitcoin doesn't change that.
(And if you're paid in bitcoin, that's bartering and you either report it or you evade it.)
If they're reporting it, sure. Companies defer profits all the time to do this within the bounds of what is legally acceptable. What's not fine is if the numbers only add up if you're not reporting.