According to the codes, which speak about "stress range" you should take the stresses over the complete range from -4 to +220 degr. C.
Personally I would use two cases with T-ambient at 24 degr C. That way you'll have the displacement figures correct.

The method used in your company, checking from 24 to 220 degr C. only, is used in a lot of other companies too. Here in The Netherlands I always see that done, instead of checking the complete stress range.