AFAIK Li-Ion batteries should never be charged to 100% because it shortens their life, most phones have a safety-mechanism to prevent overcharging.
Its intended to stop charging at 95% to 99%, but will almost always show that the battery is charged to 100% so the customer wont complain about it.
yes HTC is lieing to us, but for our own satisfaction.
So after we flashed the CWM recovery this "feature" is not there anymore and we can see the true charging state.
By plugging the power in and out over and over again to reach 100% you just damage your battery with those short charging cycles.
I read a interrestion thread about it in the Nexus S forums awhile ago and Wikipedia seems to confirm it.
I hope I could help you.