I'm sure there are different opinions on this topic.
I'd like to know if you foil, or don't foil the heat deflector and why.
I do not. Foil does affect the temperature in the cooker, and keeps the diffuser from becoming seasoned, causing it to rust.
I foil - makes cleanup of the defuser so much easier but @Yoder_Herb makes a great point and I've never questioned his opinion.
I did for about 2 years. Then didn't for a year, I don't anymore. Scrape with a metal scraper, good to go. Never rusts, and works just fine.
BTW, I have had my Yoder 640 for 6 years. And I have the two piece diffuser, so easy to clean the firepot for the next cook.