The simple answer to the question of when you should obtain a receipt is always. When you sell something or give something away which has any value, either monetary or personal, you should obtain a signed receipt from the person receiving it.
The receipt protects both parties involved. The seller, or person giving the item, has proof that it was indeed turned over to someone else. Usually the receipt states the legal names and addresses of both parties, the purchase price or conditions under which the item or money was turned over, and the date. It is quite similar to a bill of sale in showing that a transaction has taken place. With the receipt, the seller can offer proof for taxes and legal purposes that he or she sold, or turned over (donated), property/items of value, thereby obtaining a monetary deduction for the same. Receipt forms should also be utilized in landlord/tenant matters to prove that security deposits, rent and repair bills were actually honored. When obtaining a service, such as car rentals, vehicle repairs, haircuts, dry cleaning, etc., it is also important to have a signed receipt upon payment. The basic rule is that any time you wish to prove you did indeed turn over property or funds to another party, you should receive a receipt to prove that the transaction occurred.