Check, or estimate, the volume of your usual take home dose. Look up TSA guidelines. Carry on if you can, lost luggage / spilled meds due to pressure changes in flight would really suck. If you check it, seal bottles in air tight baggies or a larger bottle to contain spills. Keep the original of the physician letter on you. You will be fine. I traveled with liquid methadone a few times post 9/11 in the US. In the future, if you can, request tablets. Sure taking fifteen 10-mg tablets is a hassle, but I made out fine on a two week trip taking twenty tabs a day (my dose was 200-mg/day!)
Never an issue.
International travel can be impossible with methadone, at best it's very difficult if foreign customs finds it. In the US - you're fine. But... Have info on the closet clinic that guest doses and know their procedural requirements ahead of your trip, just in case...
You'll be fine.
For international travelers who aren't connected (corporate execs, famous addicts, etc...) bupe is the best way to travel hassle free. Methadone is just really looked down upon in most counties outside Europe and N. America, even there it's a maze of requirements to import for single personal use, and getting caught is potentially a life sentence (Malaysia, Thailand, Singapore...)
Last flight to Malaysia we were warned before landing to ditch any drugs, even prescription of it wasn't current and all info matched passport, b/c "malaysia implements the death penalty for drug traffickers."