I don't think that's necessary. Also, you could say the same thing about TrueCrypt because you didn't personally write their algorithm, did you? Also, using TrueCrypt would hinder the convenient process of syncing that an online services uses.

If the online service uses 128-bit or 256-bit encryption BEFORE your data is sent to them over the Internet, then your data can't be decrypted by using another backdoor key besides the encryption key you created. If the service does not have the key, then the data cannot be decrypted and is garbage to anybody without the key. Of course, a hacker could guess your key via brute force, but you can make guessing basically impossible if you have a complicated key that's in your brain (or otherwise not on a computer anywhere).