Do you give IV fluids to avoid renal compromise in patients with inflammatory myositis with a CK over a certain threshold?

No, because myositis is a chronic disease, CK rises slowly and typically doesn't affect kidneys (unlike rhabdomyolysis, which leads to an acute rise in CK to very high levels). However, when CK is too high like > 20,000 to 30,000 rarely in cases of necrotizing myopathy or cases of metabolic myopathy...