This tends to be something to approach with caution. If the value isn't going to have math done on it then it is probably isn't a number. A US ZIP code is technically a number value. But it has leading zeros so converting it to a numeric data type will make the data incorrect.
I'm not saying to never convert numeric type strings to numeric data types. I'm saying you need to understand your data and plan your fields correctly.
Agree completely. Some consultants not from the US wanted us to use an int field for the SSNs. They didn't know that many SSNs begin with zero. The math rule is a good one, especially for numbers that exist outside the local enterprise. I have used them for internal numbers such as SKUs where I'm assured they will always be a non-zero starting number.