With kidneys, the question seems to be, as it is with basically all drugs and supplements, "how healthy are the kidneys?".
Strontium, other supplements and drugs taken by mouth, are secreted by the kidneys. If the kidneys don't work, then drugs, supplements, and yes, strontium may build up.
As with any drug or supplement you need to be careful with accumulation if you have renal insufficiency .

The strontium ranelate site has this breakdn of use with kidney impairment:
"strontium ranelate is not recommended for patients with severe renal impairment (creatinine clearance below 30 ml/min).
No dose adjustment is required in patients with mild-to-moderate renal impairment (30-70 ml/min creatinine clearance)."
This might be a good base line for use of strontium citrate, just to be safe, but as with many supplements, no one has spent the money to check.
It is always best to ask your doctor.