Fortunately I live in a country where such thing is a big tabu. I only need to declare my gains (and only in case I'm holding less than a year) - and there are none if I am simply using BTC to send cash.
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.
If you buy btc, and value goes up, and you use it to "send cash" that is a gain.