It's not beneficial to bettas to have any salt in their tank. Period.
AQ salt can be harmful and even deadly when freshwater fish are exposed to it for any extended period of time. Even a little bit of AQ salt will in time harm.
AQ salt is used to treat external problems in fish, and only used in certain measurements and for only a specific length of time.
Exposure to salt on a regular basis will lead to kidney and liver failure. It will also cause the "bad" bacteria to become immune to it, and the fish can become immune to it.. so when you actually do need it to help with a sick fish, it won't be effective.
AQ salt and live plants do not go together and highly recommended not to place AQ salt in with live plants.
Wherever you read that a bit of AQ salt is good for bettas was completely wrong. Some people use it, but those tend not to be people who are as knowledgeable with bettas. It's best to get information from people on here such as Oldfishlady who has been breeding and raising bettas for 40+ years, MrVamp, DQ.. and even me (I only have a little over 18 yrs experience in tropical fish keeping). Unless things have changed in the last couple months since I've been off the forums... AQ salt is still a big NO-NO for healthy bettas.
*edit* was typing when you responded- woops! (at work)