You do mind but if the customer insists you have no choice.
My customers use metres, feet and statute feet, they work in degrees, deg/min/sec and grads. They work in X,Y,Z or N,E,Z (don't get me started on South Africa)
Then you have day/month/year and month/day/year, then you have Canadians that work in both.
It's easy for a web 2.0 site with no actual customers.
And Wall st would be much better if it just hired accountants instead of all those people who had spent 10years learning all that useless math.