I hate to say never, but I think the answer is never. I try to keep the loops fairly small, about a half inch to an inch at most in diameter, but with the small diameter of tippet the loops end up being more like parallel lines. The leader loop, even if it's maybe 3X, gets permanently "deformed" into this shape, so that sometime it's even difficult to slip the tippet loop through it. It's more like a slot. Now I do sometimes get my leader/tippet tangled around the loop connections at the butt end of the leader, but this is rare, and loop connections for leader-to-fly line are commonly used. And the loops don't affect my casting at all, as far as I can tell. Maybe some fly fishers with super-refined casting abilities and super-technical leader formulas would notice something, but not me.