From my personal experience, I found that my phone acts a little crazy and unresponsive when I put it on charge. Though it acts normal when it is being USB charged. I guess the difference is because of the difference in charging current it draws in both the modes.
Normal charging draws 650 ma, which sometimes causes CPU to be less responsive to avoid overheating of the system. It doesnt happen to all the phones but it surely happens to a lot of them as I found multiple posts regarding it on this forum.
USB charging, however, draws only 500 ma and hence the system is stable.
All the information here is gathered from multiple posts/threads.
ROM: C-Rom 7.1