And to reply to the original post, that is correct, you do not need to fill in a customs form if you have nothing to declare. They don't even hand them out on the plane any more. You do have to fill a "disembarkation form" where you provide some basic information about your flight and address in India.
Note - in many cases, even if you have nothing to declare, your baggage may undergo screening as you exit. The hand bags can be scanned right after you pass through immigration (e.g., at Bangalore), and any or all of your bags can be scanned again after baggage claim randomly as you exit after baggage reclaim (common at Mumbai, sometimes happens at Bangalore and elsewhere).
Regarding currency, in my experience (more than a dozen trips to India in the last couple of years) they don't actually bother to search - so if you inadvertently had, e.g., INR 30,000 on you, it's not very likely that you'll get into trouble for it. But they do seem to profile and search *some* travelers based on some heuristics or intelligence about them, because the newspapers always have reports of someone or the other caught flying in or out with large amounts of cash in various currencies.