Originally Posted by swise
In the US, it's customary to tip the taxi drivers 10-15%, all waiters 15-20%, etc. In fact, waiters and even taxi drivers actually depend on tips for their wages. Waiters usually make below minimum wage ($3 per hour perhaps), with the expectation that tips will cover the rest. If they don't get tipped, they essentially don't get paid.
Maybe the point being made was for tips for which you cannot get a receipt for i.e. cash tips?