It depends. If you fall short of the lease requirements for any reason, a guarantor can help reduce the stress of applying. Using a co-signer does not mean you are a bad tenant – it simply acts as insurance for your potential landlord if you do not meet the income level, credit score or other requirements.
Each individual property owner develops their own tenant selection criteria based on experience and characteristics of the local market. Some place heavy importance on credit score (as they feel it's a good indicator of a responsible tenant). However, some review an applicant's credit report in full to try and understand the reasoning behind a lower-than-average credit score. Nonetheless, some are willing to rent to people with a poor or limited credit history.