I would say it is very disrespectful to call someone a liar without proof. That is a character judgement without evidence.
I personally think that people should afford people a little courtesy. Avoid personal attacks. If you feel they are wrong on an issue, state why you think they are wrong. Don't call them names and insult them. Of course I can understand it in some cases, as I have been party to it enough times that I can't claim to be perfect. I strive to, though I admit the past 8 years of politics has worn me a little thin on some subjects. I say give others the respect you would want for yourself.... scratch that, give them the respect you would want them to give your mother.