underside dents from jacks
 

You know how some people put any regular jack stands on the main frame rails under the car and dents the hell out of them?

Does this cause the chasis to be weaker than others? Do people ever box those in?

No. They are known as crush rails for a reason.

I put a 2x2 block of wood right inside the rail, and jack on that. Keeps 'em from getting mangled

I always just lift from the front cross member, or the 3rd member. But that's just because I'm wierd about lifting my car and putting stress on the chassis. I have lifted from the frame rail, but I put a long 2x4 on the cradle of the jack to distribute the load evenly.