Its all in the previous posts:
- Avoid film higher than iso400
- Ask for a hand check if the airport is one of those smaller, friendly, less busy kinds. I would only do this if I expect my film to go through lots of airports (potential scans). Heathrow will scan your film multiple times if you ask for a hand check, just out of spite (true story)
- Never place film in check-on luggage. Carry-on only.
Carry-on luggage scanners have become increasingly friendly to film over years. Even compared to just 7-10 years ago. I don't think they did that for us film users though.
To get damage to iso400 film via carry-on scanners, it takes a lot of runs through the scanner. I always get yelled at for relating my personal experience, but I had damage to iso400 film from a trip about 15 years ago when my film went through 10 scanners (may have been 9). ISO 100 film showed no noticeable damage, but ISO400 (slides) had overall fog (I think it was either green or pink, but I've long thrown them out).
I'm about to fly domestically in US this Wed. I'm bringing ISO100 film and not bothering with a hand check.