Apple device use while charging?

Please.... Can someone tell me if charging my iPhone, iPad while using it be bad for the battery? Can I, please, only have answers from people who really know!!! The following anecdote will show the conflicts:

I've read forum response after response saying "NO", it causes no harm and took that as the answer. I was with an Apple store tech today who definitely "advised" that was a battery killer. I told her I drive long hours using GPS while streaming music, texting and reading books all while charging because the battery would drain in a short time otherwise.

Her response was that was one of the worst practices I could do. Well, if an Apple tech says this, who in the world are we to believe???? Someone answer!