It depends. For some visa offices like the Indian ones, IELTS is compulsory and will result in refusal if not submitted. In case they don't require it then i have seen some applicants getting approved based on a language proficiency waiver. However, if your language skills are good, then you should give IELTS because anything above 7 will increase your chances a lot and in some cases, applicants are rejected because of IElTS (i was refused ) even if they provide all the necessary language proficiency waiver, SAT and O levels English result to make up for it. My office was Abu Dhabi and i reapplied with 8 bands overall and got approved so please do not take the risk and give IELTS.