I personally increase my bet when I feel like it. For example, if I gamble with a $50 bankroll and the game yields a profit equal to that amount, in situations like this, I sometimes withdraw it all or just the initial amount, but I also sometimes increase my bet to achieve even bigger wins.
It's our individual right to gamble, whether to stick with the lowest bet or increase it. The important thing is to be careful not to exceed reasonable limits, as the consequences can be severe.