It depends on your purpose and amount of data you are sending to server.
For example, you definitely don't want to send blobs of data through GET request because, some browser has character limits over GET. Again, in case of sensitive data say, login credentials, you must have to use POST.
Following is a good post about it... Might be helpful.
http://javascript.about.com/od/ajax/a/ajaxgp.htm[
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