It is a thing you should try not to do.
It is especially important if you are doing something like adding a definition to urban dictionary, posting on a forum or adding to wikipedia etc., and would like your definition, or post, to be taken seriously and to avoid all your hard work being ruined and losing credibility.
I am not including any spelling differences that exist between the UK and the US and am only referring to 'established' words rather than urban, street, text or forum and IM languages.
You do not have to be a good speller. You can always have dictionary.com open in another window, or just type the word (however you think it's spelled) into google, and if it's spelled incorrectly, google will offer alternatives, and you can just cut n paste!
The bad spelling started me doubting the knowledge and intelligence of the person posting the definition and, therefore, the definition itself.