The following has come to mind with respect to this:
1. If you are doing Paath from Siri Guru Granth Sahib jee, then you must do full Keshi Ishnaan and wear totally Suchay Khalsa Baana.
2. If you are doing Paath from Gutka Sahib or Pothi Sahib, then it's optional to do full Keshi Ishnaan but at the very least Punj-Ishnaana is required. Some Shardhavaan Gursikhs however do Ishnaan when doing Paath from Gutka Sahib and Pothi Sahib.
3. If doing Paath orally, then there is no requirement to do Ishnaan.
4. If the body becomes dirty then nevertheless, one should take shower.
Guru Sahib knows better.
Kulbir Singh