Same tax logic should apply whether bitcoin or an alt.
In the US, the question as I understand it is whether an exchange of bitcoin for an alt (or an alt for an alt) is a taxable event. The answer is "no" if it is considered a "like-kind" exchange. Whether the IRS will consider bitcoin and an altcoin as "like-kind" is one of those unanswered questions as far as I can tell.
https://en.wikipedia.org/wiki/Like-kind_exchange
There are scenarios that can get really complicated. Suppose you buy some ether before the split and sell some ETC (or some ETH) after the split. How do you determine the cost basis?? Fuck if I know.
EDIT: Oh I think I misunderstood your question. In the US, if you exchange alt for bitcoin, and then immediately sell the bitcoin for fiat, and you assume alt and bitcoin are like-kind, then the taxable event is when you sell the bitcoin for fiat. If you assume they are not like-kind, then exchanging alt for bitcoin triggers long-term capital gains/losses, and then selling bitcoin for fiat triggers short term capital gains/losses; however, if you do those sales essentially at the same time, then you consider the price of bitcoin to the the same for each event, so your short term gain/loss is zero. So your tax would end up the same, regardless whether you consider alt and bitcoin to be like kind.
EDIT 2: You would pay zero tax since your long term cap gain rate is zero.
Disclaimer: I am neither an attorney nor an accountant. Listen to me at your peril.