That is an interesting insight on the customer behavior. In our current design we have set the default at 5% (mostly with an intention of providing a visual hint of a tip in the bill and expecting customer to change) and are seeing lot of customers don't change it to anything else most of the times. On the other side we have seen that the customers, who decide to tip in cash, slide this to 0 and pay as high as 25% of the invoice at times.