Bharti, every pranayama practitioner should imagine as you have discussed above in your question. Your question is related to the practice without kumbhaka. But by omitting the kumbhaka session for each pranayama cycle, you are also omitting the imagination associated with kumbhaka and therefore, the whole process of imagination will not be fruitful during pranayama. Let me explain in the following way:
When you inhale, imagine inhalation of the pranic energy coming with the breath being inhaled into the lungs or body. Now when you retain this breath during antara-kumbhaka, after inhalation, you also imagine that inhaled pranic energy is spreaded from the lungs or navel region to all parts of the body or even pranic body. Now, when we exhale, we emit out the negative energy from the body. It is true as you said, the exhaled negative energy will be again re-inhaled if we do not do retain the breath - bahir-kumbhaka. So, we need to retain or do bahir-kumbhaka for some time before next inhalation.
