JK herself has actually dismissed this theory on her official website - jkrowling.com, in case you couldn't guess - apparently, there are ways for wizards to get around the effects of Veritaserum if they're aware that they're going to have to take it. For instance, if Sirius were aware that he were going to be given the serum, he'd be able to prepare his mind to tell lies under its influence - at least, that would the fools at the MoM's point of view. Therefore, it's not exactly a flawless way to tell if someone's telling the truth. The only reason it worked so well on Barty Crouch Jr. in GoF is because he was unconscious when he was given the potion.