Personally, I never buy salvage title cars.
Here in CA, insurance companies tend declare a car a total loss (ie, salvage) when the cost of repair hits or exceeds 75% of the car's market value. Some insurance companies have different thresholds.
However, if the car has, for any reason whatsoever, has frame damage, it's ground for immediate total loss -- because straightening out the frame can very easily run into 5 digits (and that's if you don't section the frames from another car, but that's a totally different story).
Costs of parts, materials, paint, labor, shop fee....it adds up real fast. Hence why older mercedes whose market value are <$5000 tend to be declared as a total loss.
Now, frame damage is the REAL reason why I never buy a salvage title car. Safety is compromised, the vehicle rebuilder may have cheaped out when rebuilding the car (happens far too often), handling is different, the car drives differently, etc.