It totally depends on the pattern. When you're shaping for armholes, for example, you'd decrease at each end. Some patterns call for decreasing evenly across the row. Sometimes the decrease is supposed to show as a decorative thing.
When you're increasing for sleeves, you increase at each end, but when you are going from ribbing to the main part of the sleeves, you increase evenly across the row.
Then there's lace--a conglomerate of different kinds of increases and corresponding decreases.
So my advice would be to learn them and put them wherever the pattern tells you to.