If you're a backpacker, you likely show up in places without reservations all the time. Delhi is one place you might consider booking in advance. It's a large, chaotic city, and while you can negotiate hard and get the best price, it's convenient to know exactly where you're headed when you first get there, particularly if you're arriving on a long flight. Taxis from the airport have a set price if you arrange inside the airport. Don't go with any taxi not officially marked.

When you depart from the Delhi airport, you must have a copy of you ticket, even if it's an e-ticket. Security is understandably tight, and guards require prove that you're booked on a flight before they let you enter the airport.