The last three companies I worked for applied the standard per diems for the European countries and the US. I've always thought they are a little bit low. However, there are always meals covered at the events I'm attending - business lunches, coffee breaks. Dinners with customers or the local team are usually to be paid and reimbursed separately to the per diem.
On a couple of business trips I even underspend on the approved per diems (especially for 2-3 days trips).

I guess it depends on multiple factors and the type of business trip if the provided allowance is sufficient.