Is it mandatory to take a shower before performing Nitnem?

The cleanliness of body is required for physical health and cleanliness of mind is required for mental health.
ਭਰੀਐ ਮਤਿ ਪਾਪਾ ਕੈ ਸੰਗਿ ॥ ਓਹੁ ਧੋਪੈ ਨਾਵੈ ਕੈ ਰੰਗਿ Jap – 4
There is no hard and fast rule for washing the hair. Normally People do not wash their hair daily, but washing interval depends upon the climate and type of work. One should be fresh and awake while studying Gurbani written in Guru Granth Sahib. Hence, it always better to take bath in the morning and then study Gurbani. In case it is not possible because of medical or some other reason. Then action can be taken accordingly.
There is no specific time or place required for studying Gurbani. The requirements are same, as are required for carrying out any other study. We should be physically and mentally fresh.
ਮਨਿ ਸਾਚਾ ਮੁਖਿ ਸਾਚਾ ਸੋਇ ॥ – 272
Purpose is to study understand and follow the Hukam of Akal Purkh, so that one can become Sachyaar. Gurmat does not accept any ritual whether it is related to Path or any type of meditation. Main purpose is to recite listen understand and follow Gurbani written in Guru Granth Sahib.
ਗਾਵੀਐ ਸੁਣੀਐ ਮਨਿ ਰਖੀਐ ਭਾਉ ॥ ਦੁਖੁ ਪਰਹਰਿ ਸੁਖੁ ਘਰਿ ਲੈ ਜਾਇ ॥ Jap – 2
We should always clean our hands before touching any book. Because cleanliness is very important. The purpose of covering our head is also that the dead hairs do not fall in the surrounding. The tuban keeps our head in the normal condition both during the hot and cold climate. We need to understand and follow the Hukam of Akal Purkh and try to go in the direction of becoming Sachyaar.
ਹੁਕਮਿ ਰਜਾਈ ਚਲਣਾ ਨਾਨਕ ਲਿਖਿਆ ਨਾਲਿ ॥੧॥ Jap – 1