BTC has stayed the same price for the duration of my transfers. It's just a few hours most of the time. There is also allowance in the law for this case - I don't need to declare anything unless I profit over $1k/year.
I buy it when I need to transfer (usually using a Bitcoin ATM) and then transfer it immediately - and sometimes I deposit it to the target wallet directly.
What about for the duration of you keeping it in your wallet?