For a while now I've used a blood knot for tippet to leader knot whether I'm doing mono to mono or fluorocarbon to mono. I've found that in most cases, 4 wraps each side, for most diameter lines, seats better and isn't any weaker than 5 wraps each side. I've always heard that in scientific tests, double and triple surgeon's knots are stronger than bloods but personally (and I have done break strength tests), I've never been able to make surgeon's as strong.
For tippet to fly, I've been using an Orvis knot for about a year. I tested this as well, head to head against a clinch and an improved clinch and it wins every time and is a super quick knot. Check er out http://www.orvis.com/orvis_assets/files/index.html.
I just started messing around with another new knot that in a few tests I've done so far, has beaten the blood knot hands down. It's called the Orvis tippet knot and is on the same website. Anyways, I'm gonna keep messing around with it with different diameter line and fluoro/mono combinations and see how she works. What is everyone using these days for tippet-leader and tippet-fly knots? Thoughts on these 2? I am aware that this is incredible nerdy and no, I do not work for Orvis.