Trying to speedrun White/Cerberus/Platinum…

Nuking on white is often situational rather than efficient since there is not an ideal position to force spawns in a specific point all the time and the map is not as small as glacier. Your choice may also depend on character + weapon combination for instance. Sometimes is faster to retreat a bit, wait for other enemies to catch up and then nuke, some other times is faster not to wait and outright kill the trash spawn without a rocket (especially with the highest dps kit like GI, GTS and DAI) hoping for good adjustments between respawns and banshees approaching you (being able to stop banshees quickly is the key part). From time to time, with very unlucky situations, nuking that trash might be your fastest choice. With practice you kind of “feel” if it is worth it or not.
I don’t know If you’re asking specifically for wave 4, 5 or both since you mentioned engineers + ravagers, but as a general rule, if you are dealing with bad spawns and you’ve decided not to nuke immediately, hold the corner of win or another good spot and kill stuff till you get a decent amount of budget to nuke. Often, being patient it’s the fastest and more efficient approach despite bad luck.
Having a plan on when to nuke and how much you want to nuke in a specific wave is good, but rarely everything goes your way. Take what the rng gives you, many enemies with high budget value in front of you in a wave in which you didn’t plan to nuke? Pulverize them anyway, it might be your best chance.
Sometime you’re going to “waste” a rocket for an underwhelming spawn like the one you mentioned and you end up regretting it cause maybe by waiting you could have use it better, some other times you don’t even get a better chance and if you didn’t nuke right away, you’re going to regret that instead. I don’t think there is a systematic approach that always work, it’s all very situational and luck dependent.
That’s my personal take on it though (from a ps3 player perspective).

IIRC, Wave 5 doesn't have engineers. I'm assuming you meant centurions. They both have the same code word though - utter pieces of garbage, so there's some degree of similarity. 

As for nuking in speed runs, you should at least get one boss unit, preferably two. 

You can force the initial spawn in the basement area fairly reliably. I'd nuke the initial 5-enemy spawn at the start of the wave. (If it's a 3-enemy spawn, I'd be hesitant).

Without AC V, I'd spend two nukes on w5 - once at the beginning and the second one depends on the context. You could get lucky if the first respawn is close to you - nuke it.
Otherwise, wait for the banshees to come together (along with some other stuff, ideally) mid-way through the wave or towards the end.

If you take AC V, wave 5 deserves 3 nukes.

Also, what character/weapon set-up are you using?

^^^
Remind me never to try this Plat solo shit.

For now, learning how to nuke. In long term and with different characters, sub 20.

Nuking is more of intuitive thing.There are some rules of course.5,8 and 9 wave are a must vs cerb,but i nuke pretty every wave if i think it will help me with the time.The clean up on 3 is another good option to save some seconds/mins,but only if the spawn is really good.Watched part of your video and you nuked two time 3 goons and a banshee.Never do it.Only in desperate cases.When the atlases aren't around just stop the bashee,kill the goons and if you can kill the banshee.Wait for the atlases.In some cases nuking depends on the chracter as well.As @scarletj5 mentioned use more OP chracter just to get used to it.AIU is not that fast killer despite the good shotgun passives and cloak bonuses.Everything comes with a practice.

I don't recommend this with slow chracters tho.It's absolutelly luck based nuking .The best way is try this,but if the spawns aren't good,just stop the banshees,wait for some more nemies and then nuke.By this way you will need only two 2 rockets as well.

Been practising W8 with QMI and it looks like I can reliably hold it without nuking or spending too much ammo. Should I still or it's better to save more nukes for Banshees?

At least 1 rocket is a must on 8 wave.

Forcing spawns on firebase White doesn't work good,especially on 8 wave. Sometimes you can catch all of them ,but most of the time the spawns are crap. Just wait for more units