Is CTS necessary on a AES-CBC DRBG? As I understand it, CTS pads the last block and swaps it with the second last to compensate for a partial block of data. I have written a DRBG (Deterministic Random Bit Generator) using AES-CBC that only accepts an input size which falls on the block boundary. 

Is CTS really necessary here? I can't imagine a way in which it's absence will affect the entropy of the output. Am I understanding this correctly?
Normally you don't want to reverse the encryption used within a DRBG. Schemes like PKCS#7 padding and CTS are required to deterministically reverse the padding during decryption. If you just want to have a final block, you should be fine by using zero padding until the end of the block.

Of course, if this zero padding is actually harming the output of your DRBG depends on the algorithm.