Depends whether it's a major 7th or a dominant 7th or a minor 7th.
Also depends which degree of scale, and there are lots of exceptions.
Most of the time I use the 7th which fits the key/scale/mode I am using.
Whenever I see dominant or minor chord in a piece of music, I tend to extend them to the 9th, mainly because I like the sound of the 9th and because the voicings are usually easier on the guitar.
My most commonly used voicings in the key of C major:
For the I chord (Major chord): C6, C6/9, Cmaj7, Cmaj9, Cmaj9#11
For the ii chord (Minor chord): D-7, D-7b5, D-9
For the V chord (Dominant chord): G7, G9, G13
To the OP, the way I learned upper intervals was:
6th's: The 6th is a prime interval which denotes the 6th degree of the scale. [R-3-5-6] It is used in major and minor-like chords, but not in dominant chords which are generally indicated as the upper interval 13th [R-3-5-7-9-11-13] which is one octave above the 6th interval. With min6 chords you don't sound that min7 because it will clash, but sounding the maj7 interval is fine. One of my favorite chords is a C6/9 [R-3-5-6-9]
9th's: The 9th is the upper interval which denotes the 2nd degree of the scale one octave above the root [R-3-5-7-9] and is a third above the 7th. 9th's can be used on ma7, dom7 and min7th chords. The cool thing about this is with major and dominant 9th chords, the 9th can replace the 3rd. Even if the 3rd is not present in the voicing, it is implied nonetheless.
11th's: The 11th is the upper interval which denotes the fourth degree of the scale one octave above the root [R-3-5-7-9-11] and is a third above the 9th. 11th chords usually include the 9th. With dom11 chords, we can omit the 3rd to to avoid clashing with the 11th, but min11 chords must include the 3rd. It's common to sharp the 11th on major and dom chords to avoid clashing with the natural 11. I never use dom11 or min11 chords, but don't let that stop you.
13th's: The 13 is the upper interval which denotes the 6th degree of the scale an octave above the root [R-3-5-7-9-11-13] and is a third above the 11th. The 13th is mostly used in dominant chords, but can be used on minor chords whenever. With 13th chords, the 3rd is often subbed out for the 9th but not always. You can dump the 11th if you want. I usually do.
