I was wandering about that. It always seemed the last few minuscule % needs more full rolls than they should. I tried to push to 100% in the beginning then realized the numbers are not that different. If the increase was insignificant I just stop there now usually it is at ~98-99%. Thank you for confirming it.
Yeah, there's very rarely any point maxing out a G5 unless you have a lot of spare materials. Something else to remember is that - unless you are doing something which needs massive optimisation - Grade 4 is massively cheaper in terms of materials than Grade 5 and still really good.
A maxed overcharged Grade 4 multicannon does 20.2 DPS. Maxed at G5 it does 21.4 DPS. Is it really worth about 10 Modified Embedded Firmware, Conductive Polymers, and Zirconium for that extra 6% damage on one weapon? (Or about ~1% extra total DPS across your entire ship?)
A 7A shield generator on an Anaconda gives you 595 MJ. With reinforced G4, this goes up to 785 MJ, or to 821 MJ at G5. Is it worth 10 Improvised Components for 5% extra shields?
On a mid-weight Python, Grade 4 Dirty Drives (+drag experimental) gives 364m/s cruise and 474m/s boost. Going up to Grade 5 gets you 381 cruise and 496 boost - again, less than 5% extra speed, at the cost of 10 or more Pharmaceutical Isolators.
For PvP combat, or racing, or trying to explore the furthest reaches of the galaxy ... yes, that extra 5% can be worth it. For just wandering around trading, mining, and doing a bit of light PvE combat ... it's probably largely unnoticeable in practice. Lots of people would find engineers far less grindy if they looked at what engineering they actually needed first, rather than just going "must have best".