When a score calls for a triad, it should be played as a triad, without a seventh. Adding the seventh changes the sound and function of chords and risks clashing with the melody.

It is often okay to add a sixth to a triad, for color, because adding a sixth doesn't change the nature of the chord in the way a seventh does.

If the performer judges that a seventh can be played where a triad is specified, then one can follow @Tim's guidelines for major keys and the following for minor keys:

Minor key seventh chords

i = m7 (or mM7 if the seventh is raised)
iio = 7b5 (half-diminished)
III = M7
iv = m7
v = m7 (or V7 if used as a dominant chord / with raised seventh)
VI = M7
VII = 7 (i.e., dominant seventh)

Be careful, when adding sevenths, that you're aware of any modulations. For example, if one is in the key of C major, but encounters the triads F and Bb at a cadence point, then the minor seventh would be added to the F chord — creating an F7, the dominant of Bb — rather than the major seventh, which would otherwise be consistent with C major.

Direct answers

Guide tones:

Major and minor triads generally are not thought of as having guide tones in the way seventh chords are, since they are considered stable within themselves; whereas seventh chords are considered inherently unstable and needing to resolve. The guide tones are the notes that most typically are the primary points of resolution: the third and the seventh.

To the degree there is (contextual) tension in a major or minor triad, the third of the chord is the first place to look, as it will often be the leading tone of the following chord.

Does adding a seventh change the function of a chord?

One should proceed as though it will. Turning a major triad into a dominant seventh, for example, potentially signals a modulation. It might or might now work depending on the musical context. Similarly, turning a dominant V chord into a major seventh weakens its function as a dominant chord.

Triads and seventh chords as separate entities

Music theory considers triads and seventh chords as separate entities. Further extensions (9ths, 11ths, 13ths) are generally interpreted in terms of the underlying seventh chord.

Detail

When a score calls for a triad, it should be played as a triad, without a seventh. Adding the seventh changes the sound and function of chords and risks clashing with the melody.

It is often okay to add a sixth to a triad, for color, because adding a sixth doesn't change the nature of the chord in the way a seventh does.

If the performer judges that a seventh can be played where a triad is specified, then one can follow @Tim's guidelines for major keys and the following for minor keys:

Minor key seventh chords

i = m7 (or mM7 if the seventh is raised)
iio = 7b5 (half-diminished)
III = M7
iv = m7
v = m7 (or V7 if used as a dominant chord / with raised seventh)
VI = M7
VII = 7 (i.e., dominant seventh)

Be careful, when adding sevenths, that you're aware of any modulations. For example, if one is in the key of C major, but encounters the triads F and Bb at a cadence point, then the minor seventh would be added to the F chord — creating an F7, the dominant of Bb — rather than the major seventh, which would otherwise be consistent with C major.