A recently released study indicated that people who swear are generally more honest than people who do not. This could be because people who swear are less inhibited in their interaction with others and less likely to filter information. It also may be because they are more up front and forward about how they feel. Either way, data would suggest that they are more honest.
Swearing does make any person more honest, if they were thinking it. Swearing should not be done by someone just to look more real though. Cussing and profanity should only be used by adults around adults in the right situations, but if a dirty word crosses your mind, you shouldn't bury it just to fake being civil and polite around others.
Swearing makes someone neither more or less honest. The words are simply an additional way of expressing a feeling or adding emphasis. Abstaining from using swears is the same thing, it says little about how honest a person is. Swears should be ignored completely when deciding the honesty of words and instead the focus needs to be on the rest of what is said.
Swearing is a way of expressing negative thoughts in a brute form. While swearing leaves no doubts as to what the swearing person thinks, there are much better forms of expression. Honesty does not have to take the primitive form of swearing. Any negative thought that can be expressed by swearing can also be expressed using acceptable words and a calm and respectful tone.