The chemistry in these batteries are such that keeping them at 4200mV (which is full charge) for extended periods of time will cause their maximum capacity to decrease. This is due to oxidation within the battery cells. Will this happen over night? No. Will it happen over 1 year of constantly doing this, I 100% promise you it will.
I have a PhD in Inorganic Chemistry and 12 years of working on battery technology at General Motors to back that statement up. Now, is that a big deal since you can go replace that battery for $40 at your Verizon store... Nope. I say go for it.. But expect that buying a new battery is in your 1 year (or so) future.