I wear 634 in size 38

But I bought IH-888-XHSib - 25oz Selvedge Denim Medium/High Rise Tapered Cut Jeans - Ind/blk in size 36. it fits me at second top bottom and the guy at the shop said it will strecht out with wear. Top bottom fits after second wear
it's not been washed yet but I read that it will shrink.
Is it bad decision?