It's automatically hazardous to give someone a false map of the world. If you do it knowingly you have the responsibility to make sure no harm comes of it. Even if you take that responsibility seriously, and are competent to do so, taking it secretly without consent is an ethical problem.

My take on this:

• Few people take that responsibility seriously or are competent to do so, or are even aware that it exists.
• Most of the time people's intuitions about minor well-intended deceptions are sufficient to avoid trouble.
• If you call someone a liar, that has a strong negative connotation and social implications for good reason. We didn't evolve the capacity for deception primarily to hold surprise birthday parties for each other.

There are no dirty words, but there are inaccurate ones. Use with care.