I don't think there are any rules. Or actually, I hope there are no rules, it's just a matter of taste and getting used to. Maybe someone dislikes maj7 chords, because they heard someone else say it sounded like elevator music, in a way which implied that it's a bad thing... and so, "maj7 = bad" for that person.
Music students tend to have a period in which they add extra notes everywhere, weird inversions and chord substitutions, weird time signatures, weird sounds, weird lyrics, weird clothes and hairstyles, weird everything. It's an important phase in developing a taste.
Play a lot of songs and try all the seasonings. Listen to it the next day. Do you still like it? Does it feel tasteless and kitchy, or just better? Listen to jazz and try to mimic what you hear. Can you reproduce the feelings, and does it make your playing feel credible and interesting and better, or does it sound like you're pretending to be something in a pathetic or ridiculous way? Do you think the added or changed things support the song and the artist and the genre, or is it just a gimmick?
As a guiding principle for getting ideas for jazzifications to try, voice leading is one alternative. Or you can use various substitutions like C7 => Gm, or C7 => Gb13 (tritone substitution), or C7 => Edim7, or C7 => A/C and F#/C. Try holding a pedal tone. Try everything.
You're developing a taste, it takes time and practice, but it's important to do, because it's part of what makes your sound.