So far as I can tell, those measurements only happen once every few minutes and involve turning on some green LEDs on the back of the phone. The power draw is apparently negligible. Conversely, if you're constantly waking up the screen to notify you about an email, that's going to add up. Just switching off those notifications will save you a bunch of power.

Also, FWIW, I've been continuing to pay attention every evening to the battery remaining estimate plus time used in the day. In total, I'm seeing between 28-30 hours of battery life per charge. And this is with "ambient" mode turned on. Translation: even if you forget to charge it at night, you'll hopefully notice it in the morning and you'll have the time to put it on the charger before you head out for work.