I was going to play with this, since I've never really done it before. My question is do I have to reset the AI each time the weather downloads, or just the first time?

What is 'this'? What AI (addon)? What weather (addon)? I haven't got a clue what you are trying to do...

I presume you mean Active Sky, which has an option to refresh the AI. It should refresh when you load up the sim but it's a good idea to let the weather inject and then refresh the AI so they use the right runway for takeoff. You shouldn't have to refresh the AI for subsequent weather updates.

I reset AI after initiating the first weather load at the start of the flight (I use FSUIPC traffic density toggle function for the reset -- works regardless of what weather engine you're using). The other time I might reset would be when starting my approach, it depends on how far from the airport your weather engine does its last update. I use OpusFSX and the latest release is supposed to have destination weather stabilized when you're within about 200 miles, so in my case I haven't bothered to reset AI except at the start of the flight.

Just one time after you start a flight and your weather is loaded. Please note that any plane(s) already in the landing phase will continue to their original runway and that winds 6kts or less are considered calm and the default runways will still be used.