Honestly I haven't seen a significant difference in my phone battery level when I plug it in each night, maybe 5-10% less than usual. I've been consciously trying to rely more on the watch lately for lesser tasks rather than continue defaulting to using my phone, and so I like to think my phone usage is down which helps negate any extra battery drain from the constant pairing.
The big difference has been that I never used to have bluetooth on and now it's on all the time, so perhaps that has contributed more to the battery depleting faster too.