You're encrypting in CBC or ECB mode, and the length of your data to encrypt is always a multiple of the block size. You would like to avoid padding because it adds an extra, unnecessary block of output.
OpenSSL has a function that can turn padding on and off for a context object:
int EVP_CIPHER_CTX_set_padding(EVP_CIPHER_CTX *ctx, int pad);
Particularly when you are implementing another encryption mode, you may always be operating on block-sized chunks, and it can be inconvenient to deal with padding. Alternatively, some odd protocol may require a nonstandard padding scheme that causes you to pad the data manually before encryption (and to remove the pad manually after encryption).
The second argument of this function should be zero to turn padding off, and non-zero to turn it on.