some companies (BRITAX) state that it will void the warrenty... but we're talking about warrenty should the seat fail in a crash or the child be ejected or something... not basic repairs.
now I've actually seen and experienced those strap covers (so many have used them and I've gotten to see their effects and such)... they really are thicker than they seem and the width is a problem with keeping the belts on the shoulder properly on some seat.
I'd recommend taking out needle and thread or Ye Olde Sewing machine and two socks.
cut off the toes... slice along the length... seal off the ends since you worry about fraying (but in my experience it just doesn't happen), and then take strips/circles of velcro (you can buy them in the craft/sewing section at the large walmarts)... and use that to fasten them around your straps.
Because you have to do it this way, you may want to use larger socks than the ones your baby wears, just depending on baby's shoe size.
So if you use, say, one of YOUR socks... just cut it shorter... you want it long enough to offer protection to the neck, but short enough not to interfere with the chest clip.
HTH!!