I always kept a bathroom towel between their seats and the leather which protected the leather from the car seat and caught all the crumbs for quick cleanup. I remember going to some car seat safety check events and was never told that what I did was a bad or dangerous idea. Now, thing may have changed and it is possible that each state has different requirements so I'd definitely research into the subject further. But in my experience, my seats were in there so very tightly (I sat in them while I set the safety clips) because of the safety clips that they never budged until I pulled them out.
As an aside, when the kids were about 3 they drew on DH's car's beige leather seats with pen. A little Oxyclean and a q-tip got it out

and DH never knew