akal takht has issued a "sandesh" on this issue. sikhs should donate blood and organs because it is a great seva for society, and we do not need our body after death. the body is dust after the soul leaves it. this is why we don't have so many post death rituals as other religions.
ਜੇ ਮਿਰਤਕ ਕਉ ਚੰਦਨੁ ਚੜਾਵੈ ॥
If a corpse is anointed with sandalwood oil,
ਉਸ ਤੇ ਕਹਹੁ ਕਵਨ ਫਲ ਪਾਵੈ ॥
what good does it do?
ਜੇ ਮਿਰਤਕ ਕਉ ਬਿਸਟਾ ਮਾਹਿ ਰੁਲਾਈ ॥
If a corpse is rolled in manure,
ਤਾਂ ਮਿਰਤਕ ਕਾ ਕਿਆ ਘਟਿ ਜਾਈ ॥੩॥
what does it lose from this? ||3||
image of the "sandesh":