Every time you sell coins for fiat it's a taxable event. Doesn't matter if the money stays on exchange or is re-invested or not.
Pretty sure under normal circumstances in the US we don't have to pay any tax on our net worth or anything like that. It's only when you earn money by working or by selling something for more than you paid.
But don't take my word for it. I'm just a random guy on the internet.
P.S. I didn't vote.
but the question is how to define "sell"? what if i used my bitcoins to purchase $1000 in gift cards? does that mean i sold my BTC? i'd think the more correct term would be exchange your bitcoins for fiat.