You do need to be wary of using live tubifex, they are notorious for carrying nasties which can make your fish quite sick (and often can kill your fish). I used to get live portions regularly without problems. Then I finally got a bad batch and wiped out a tank with them. Since then I've been afraid to try live again, just don't want to chance it. The frozen variety are quite safe though. Some of my bettas will eat the frozen variety, some won't. But I do find that it tends to foul up my water more so than other frozen foods.
I have some Wardley brand, freeze dried tubifex worms that are formed into a cube. I got them super cheap at Target. I usually cut the cube up into smaller pieces, at least 4, sometimes 8. My female bettas love them, but for some reason my male bettas won't touch em. All of my other fish love em.