In your harmonizations of melodies, it is best to use seventh chords to add tension as you approach tonic function. Therefore, sevenths are added most frequently to chords of dominant function ($\left.\text{V}\right.$ and $\left.\text{vii}^{\circ}{}\right.$) and chords of pre–dominant function ($\left.\text{ii}\right.$ and, to a lesser extent, $\left.\text{IV}\right.$). (See the harmonic flowchart.) For this reason, seventh chords like $\left.\text{iii}^{7}\right.$, $\left.\text{vi}^{7}\right.$, $\left.\text{IV}\text{M}^{7}\right.$, and $\left.\text{I}\text{M}^{7}\right.$ are less commonly used and encountered.