When you are doing Seva of Guru Granth Sahib Jee, i.e Parkash Seva, Sukhasan Seva, or sitting on Tabia. You should be wearing clean clothes and should have done full ishnaan. You shouldn't sit on tabia with clothes you have worn outside. Many Gursikhs have special Bana thye keep clean that they use just do seva with. Refer to Bhai Randhir Singh's article in Gurmat Parkash Pages 128-130:
11) When someone sits on the Tabia do Paat or take a Hukam it is important that they do Panj Ishnaan before sitting down. This is a sign of complete and utmost respect. Those that quickly sit on Tabia without doing Panj Ishnaan can not be called true and caring Sikhs. To sit on Tabia without washing your feet, the same feet that have been in shoes is not appropriate. This is a very heavy disrespect of Guru Sahib. To wear a dirty Pajamee and sit on Tabia is a horrible offence as well. A Singh should take off his Pajamee before sitting on Tabia. Those that don’t take off their Pajaamees are scared because either they are not wearing proper Kachherai or they aren't wearing Kachherai at all and are wearing something else. So for this reason out of fear they do not take off their Pajamee.
12) For Bibian the Hukam is before sitting on Tabia they should put on a new and completely washed and clean Pajamee or Salwar. Do not wear those Pajameea that have been worn and are smelly. They should also wash their feet. Any Salwar or Pajamee that has touched a shoe should not be worn when sitting on Tabia.
We have seen that at Sri Amritsar Darbar Sahib the Sevadaars do not take off their Pajameea when sitting on Tabia. But the fact is that they have special Pajameea that they only wear when they do Seva. After they are done their Seva they take off those Pajameea and wear different ones. In this way only clean and new Pajameea are worn when they do Seva. That Sevadaar that does not wear clean Bana when sitting on Tabia is a Daindaar to Guru Sahib. Those that wear dirty clothes when sitting on Tabia their Bhagtee will never completely come to fruit.
We have seen some Naam-Dhareek Sikhs that wear smelly clothes and have smelly feet that have been in shoes and without any fear sit on Tabia and do Paat from Sri Guru Granth Sahib. From their feet there is a strong repulsive stench that comes from the leather boots that they wear, but still with such smelly clothes and feet they sit on Tabia and do not care. Yet they claim and call themselves to be complete and perfect Gursikhs of the Guru. These people say with complete faith that without doing Darshan of Guru Sahib we do not eat or drink or start any task. We are Kurbaan and sacrifice ourselves to this commitment of theirs but alas if only they kept the Rehat that we have described above.
ਸਤਿਗੁਰਬਚਨਕਮਾਵਣੇਸਚਾਏਹੁਵੀਚਾਰੁ॥