You already have lots of good and detailed answers, so I’ll just throw in my own take on this.
- NEVER level 1* troops, they are good only as food and that’s it
- 2* troops can be worth leveling since the food cost is reasonable, and you need a place to dump your 1* feeders, however:
- Level 2* troops with 1* troops ONLY because you’ll want to save all the 2* and 3* troops for 4* troops
- Don’t bother with 3* troops unless you’re absolutely sure you want to become a 4* tourney/event specialist. The upgrade costs starts to get quite high (not as bad as 4* but still substantial) and they’re NOT in any way game changing
- 4* troops are sub-divided into 3 classes: crit, mana, ninja, you should prioritize mana where possible since these troops become absolutely game changing starting as low as lv 5 (for an average speed hero with max costume bonus, 10 tiles → 9 tiles).
- Ninja and crit troops can each be leveled to lv 9 because you pick up 2/3% stats increase on atk/def which IMO is tangible, but mana should still be priority regardless
I’ve never leveled a single 3* troop, and I really don’t feel like I’m handicapping myself or missing anything by doing so. Up to now I haven’t leveled any 2* troops either, but this is about to change as I rack up ludicrous amounts of 1* feeders, and my mana troops are all past lv 9 at this point which is way too slow and expensive to feed with 1* troops.
Slightly OT wall of text @Guvnor
Great mind thinks alike! 
I leveled and emblemed 5 copies of costume Rigard precisely because I also wanted a cleanser on each war team (6th war team has Garnet so cRigs not strictly needed), and I tend to run 4-1 most of the time so he fits in well. Troops are arranged in such a way that guarantees the 4tack will fire in 9 tiles, even if they aren’t the most optimal (eg giving Buddy a lv 17 troop when it could benefit a sniper more)
Far as troops go, I stop my crit at lv 9 because the 1% extra crit change just doesn’t make any different to me. But the other stats increases at lv 9 are quite tangible: +2% atk, +3% def, +1% HP, which is why I think it’s worth leveling one crit troop to lv 9. They’re great for using on fast support heroes who have their stats distributed all wrong (e.g. Panther, Mist, Brynhild, all support heroes who aren’t really damage dealers but for some reason have stupid high atk and ludicrously low defense).