If you are doing a true high-strength tieoff, it should not matter as there should be no tension in the knot or biner itself. The idea behind the high-strength tieoff is that there are no knots in the system to weaken the anchor. Carabiner on a 8 bight is a hell of a lot more convenient than tying a rewoven 8 around the working line, and prevents nylon on nylon rubbing when weighted (as in a highline, etc).