The AWS's official docs state that the availability zone must be the same. The region is, for example, us-east-1. The AZ is us-east-1a. So, according to AWS, it has to be specific to the last letter.
However, some clever folks found a potential work-around that uses EC2 Consolidated Billing and combines the (odd?) feature that one's us-east-1a might be another us-east-1c (I guess the idea there is to enforce some homogeneity around the AZs).
