Remember, almost everyone who uses high or even medium light also turns off the CO2 at night. This allows the CO2 in the water to outgas, leaving the pH around 1.0 higher than it was when the lights went off. When the lights come back on, or an hour or so before, the CO2 is turned back on, and within a half hour or so the pH is back down by 1.0 or so. That clearly shows that big pH changes are irrelevant.
When you don't use CO2, a big pH change can be a symptom of problems with the tank water. But, ordinary good tank/water maintenance eliminates that problem, so monitoring pH except as a way of verifying that you are getting CO2 dissolved into the tank water is of little value. My present tank setup, now about 6 months old, has never had the pH measured.
Your specific question was about dipping the test kit test tube in the tank to get a sample to test, and whether or not that changes its CO2 content. I haven't tested that, but intuitively I know it does, but probably not fast enough to make your pH measurement incorrect by enough to worry about. A possibly bigger concern would be that the CO2 concentration, and pH, is not the same all over the tank, and, for most tanks with good surface ripple, the surface water would be at the lowest CO2 concentration, so the pH of that water wouldn't be at all typical of the whole tank. If Plantbrain reads this, I'm sure he has tested this and can tell us how big this effect is, if it even exists.