According to the Information for Candidates booklet (2013) available directly from the IELTS.org website, writing in all caps (otherwise known as block letters) is an accepted exam practice:
The above matches the advice published at the British Council website:"You may write your answers entirely in capitals if you wish.”
Source: https://www.ielts.org/pdf/Information_f ... ooklet.pdf
I could not find a reference at the IDP website, but David Park of IDP Thailand wrote the following (dated 2007, but still publicly available on the Thai IDP website) that seems to suggest avoiding all capitals:“You may write entirely in capital letters if you wish.”
Source: http://takeielts.britishcouncil.org/pre ... est-advice
“…it's very important to capitalise words correctly in the writing module.
If you do not capitalise words correctly in the writing module, you could receive a marking penalty for "Grammatical Range and Accuracy", one of the four sub-skills scored by the examiner when assessing your writing.”
Source: http://www.idp.co.th/IELTS/A_WhenMustCa ... itals.aspx
