Kathy

You will level up most skills regardless how useful they are for the combat power padding in endgame. As a general rule of thumb:

• Units you are not sure you will use long term: Skills to level 4. If they're passives that benefit the team, can take that to 10.
• Strong units but not your main personality: Skills to level 10 for raids and elemental content. Not worth going higher if they are not your main team.
• Strong units on your main personality (and waifus): Max out skills. You can start with 10/10/10 and slowly work towards 12/12/12 when you can sweep higher stages.
• What about Yearnings? They unlock skills up to level 15. SUPER EXPENSIVE, only worth for your main team or really meta characters.