What's BS? If you mean I don't then I'm afraid your wrong, don't judge everyone by your own standards. You may not be able to price a job properly but others can.
It goes without saying that occasionally something will crop up that wasn't accounted for, but it's rare. When it does then that's not the customer's fault, it's either a mistake, lack of planning, or something I should have known about and accounted for. I take my professional reputation very seriously, and consider it a point of principle to be good to my word.
I can hand on heart absolutely guarantee that I have never once increased a quoted price.
@Nisus has been working with me on a load of jobs over the last three years and he can verify it hasn't happened in that time. The only time a price increases is when a customer asks for something additional to the original quote.