Depends on the root of your honesty. Is it for their benefit or yours? Is your truth objective or subjective?

Telling uncomfortable truths is a choice and does not always serve good. We are not simply logical beings but also emotional and spiritual. 

Certainly telling a happy burning person they are on fire is for their benefit. 

Alternatively, informing a confident stupid person who believes themself smart would be more for your benefit than theirs.

Informing a person that their child was sired by another man may be perceived by you to be of benefit, but a family can be destroyed and the acting father may lose a sense of connection with the child in the aftermath.