Well, it depends...
CAN Cr be harmful when on? Yes.
Can Cr NOT be harmful when on? Yes
Is Cr worth it while on? That wasn't your question, so that's a different thread/topic.
Can Cr be harmful when on? Yes... BUT, really the most simple yet best thing one can do for their kidney's while on and with Cr is
HYDRATION... FLUSH those puppies!
Just the other day we had a patient with rhabdomyolysis... basically a severe injury to the muscle(s) which results in enzymes, electrolytes and myoglobin tstart pouring out of the muscle into the circulation... if these levels go too high, the patient can go into acute renal failure and be on dialysis temporarily to get rid of the waste products. Why? The kidneys basically get "gunked up" (that's a fancy medical term don't ya know

) and can't filter.
How did we treat this patient (and is the first line of treatment for similar patients)? We doubled her IV fluid requirements with some bicarb in them to help neutralize the blood and just flushed, flushed, flushed... that's it... and time (A LOT of time).
Don't wait until you are thirsty to drink water... there is a delay between your brain and your thirst- you could be 10% dehydrated by the time you feel thirsty (enough to significantly affect training as well)... you want your urine to be nice and clear by the afternoon if not by the end of the day (morning is OK to be yellow).
- Elevated creatinine levels: well, if you are training and on Cr, then really, it's a false elevation, or an "induced" elevation... not one d/t poor renal function... if worried, just go off the Cr a week before your blood test.
Blood pressure: keep this in check to take strain off the kidney's that might otherwise impair their filtering.