Apologies if this has been asked before but didn't see a definitive answer after a quick search. In one of the Tips and Tricks threads it said that you should use lvl 5 Manyus until lvl 30, lvl 20 until max and lvl 100 the rest of the way. Does this mean you should start using lvl 100's on a newly promoted character right from the get-go?

Well in fact you can use them as you want. It's just adviced to use them like:
lvl1 to lvl30 ->lvl5 manyus
lvl31 to lvl50/60 -> lvl20 manyus
lvl51/61 to lvl 70 ->lvl10 manyus
regardless to the evolution or not.
But nothing forgive you to use lvl5 manyus on lvl >60 units or lvl100 manyus on lvl1 units as long as you how to manage coins (don't do this if you always run out of money).

If you can, then go for it as levelling them as high as possible from level 1 results in you saving the most coins then leveling them at higher levels.

That is, if you care about saving coins that is.

Do note that you'll eventually be swarmed with level 5 manyus and you'll either have to take the increased cost for less leveling, or throw them away to make space. So I'd use as much level 5 manyus as possible.

As everyone above have pointed out, it is all about saving you coins leveling your knights.
example for leveling

Level 1 knight, each Manyu cost 100 coins to use for upgrade. if you use 10 level 5 Manyu's, it will get t you yo level 23, and cost 1000 coins
If you use higher level Manyu's it will cost the same, but get you higher level's

Level 23 knight, each Manyu will cost 1670 coins to use for upgrade.

Level 69 knight, each Manyu will cost 6900 coins to use for Upgrade.

So it's all about IF you going to use all the Manyu;s you collect, then use the lower level ones up early, and then higher level Manyu's at higher knight level.

If you want you only use Higher level Manyu's to save money, but your Knight level progression will slow down.
So Most people follow the suggested Manyu usage, untill they have excess Coins, ie higher level players.

1. Expand inventory slots
2. Spread your low level manyus evenly across all your teams
3. ???
4. Profit

Seriously, save those level 100 manyus at level 50+, in order to reduce the cost of leveling up (exp to cost ratio) When you first start this game, strategically and patiently level up ALL YOUR TEAMS evenly. Don't bone up on just one team and then expect to clear maps that way.

The big key here is to make sure you have ample inventory space. Spend those flower gems to expand inventory more than to roll gachas. You'll get tons of 5★ girls from events (and event repeats) in no time.