You shouldn't trust the cloud providers. Even if the CSP and its employees are trustworthy, if they get a court order or double-secret-probation security letter, they have to turn the data over.
Whether that matters or not depends on what you are doing with the cloud though. If you are using cloud storage as a "big scalable drive in the sky", then you just need to encrypt the data on-premise where YOU control the encryption keys. Server(cloud)-side encryption helps with hackers, but not against three letter agencies.
Just using encryption to transport the data isn't enough. The data itself needs to be encrypted before it goes to the cloud. As long as you do that, you can take advantage of the cloud providers cost structure and save yourself some significant $$$ without risking your data.