Windows Azure Storage Client Library provides overloads for binary and string that handle encoding for you. As such you can make use of any serialization mechanism you like, given that the serialized form is less than 64 KB.
Hence, the answer to your question actually depends on your specific scenario. Handling JSON data would be much easier, but if you have a specific need to send the data in another format, please consider such alternatives. For larger scenarios some users augment queue messages to simply point to blob or table storage as a more flexible and verbose option while using the queue messages to provide for reliable message delivery.