After fly fishing for so many years, you'd think you have your casting down to a science. Fact is, you may have developed a lapse in your casting concentration. Don't be alarmed, this happens, especially when casting light lines during wind gusts.
In my opinion, smaller tippet, 5x or 6x, should always be changed unless the knot if very loose and easy to untie with your fingers.
There is debate whether you should untie wind knots, leave them or replace the tippet. Some fly fishers would never cast a line with a knot in it and will always untie wind knots. Some will argue that untying it makes the line weaker, so they leave the knot. Others choose to change the tippet outright.
This is what I do. I untie knots that are not too tight, by using the tip of a hook to loosen it, then check the tippet for nicks by running it between my lips to feel for abrasions.
When I'm fishing for larger fish, I change the tippet to avoid casting a weak the line and loosing my fly and a trophy fish.
I hope this answers your question. Read informative fly fishing books, watch videos and look online for resources that will help you develop the skills to become a good fly fisher.
If you watch an angler that seem to have good casting skills, don't be afraid to ask them for advise. Most fly fishers will be more than glad to help you.
May all your flies land in perfect presentation!
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