A receipt is not necessary for a Best Buy warranty. However, keeping the receipt is advisable, because the receipt makes it easier for Best Buy to find a customer's warranty.
Best Buy warranties, or Product Protection Plans, are serviced by Geek Squad, an affiliated technology support company, which sells its Geek Squad protection plans exclusively at Best Buy.
The Geek Squad website asks that each customer have his receipt handy when calling about a purchase covered by a Geek Squad protection plan, or when visiting a Best Buy store for covered services. However, the website also explains that if a customer cannot locate the receipt, the customer can call Geek Squad 24 hours a day, seven days a week, to obtain the necessary identifying information about the Geek Squad protection plan.