Repair costs estimate when offering price

[John Matthews]

When setting a maximum offer price, it's customary to assume some estimated repair cost, correct? 

I assume it's generally a good idea to see the property (and thus have a better idea for repair estimates before you place an offer? Is it appropriate to bring a contractor along to help in estimating these costs?

If you can't get into the property, how do most of you estimate repair costs to factor into your offer price?