I've been worrying alot about taxes in cryptocurrencies (especially when trading them on different exchanges) It was never about that I would lose some of my gains I've made but to not get in trouble for not declaring them the right way. I found this site https://bitcoin.tax/
it's free up to 100 transactions, but if you've done more (like i have) it costs you 20$/year to perform unlimited transactions.
You could simply import all your orders from different exchanges and get a complete document. This may differ based on where you live, but for all who is unsure about this at least do some research, this might be a great tool.
Please share your knowledge and experience with taxes in this new hot world of cryptos.
Be safe. :)