If you bought your bitcoins when they were cheaper then now then your gain is not taxable as it is an unrealized profit:
http://www.investopedia.com/ask/answers/06/realizedprofitsvsunrealizedprofits.asp
Only when you sell them you need to tax the difference because it will be a realized profit.
This is valid for any asset.
You can select when and where will happen this event and create the proper environment to optimize your tax.