I'd say almost never! because, once you get in a habit of watching them jump and go nuts for them, you'll get addicted! TRUST me.
my lps did it, and thought they were healthy because they were eating them, but they are all really sick now.
But, honestly, I do sometimes give them some. Best to stick to stuff actually recommended to bettas.
Blood worms can actually cause weight loss.
The only reason I speak so strongly of this is because my lps did it, and thats ALL they fed them, and it made me angry >:(.
but as a snack its fine.