I contacted Cambridge Test Center for the certain answer and as it may be the question of many others I put their answer here.
"Dear Sir or Madam
You may use either upper or lower case on your answer sheet. You will
not be penalised for how you write an answer provided it is spelt
correctly.
You are not, in fact, required to use upper case for proper nouns,
although it is obviously good practice to get into this habit.
I hope that this answers your question sufficiently.
Regards
IELTS Exams Administrator
Cambridge ESOL
www.ielts.org"