Every Slot which has an item-level of 589 or less will get boosted to 590. And everything from 590+ will keep its item level.
This in mind makes 91, 95, 97 and 98 a viable twinking option.
91 -> 3 640+ items warlords crafted and 620 alchemy trinket.
95 -> 3 640+ items warlords crafted, 620 alchemy trinket, 2 random class perks and all other slots 600.
97 -> 3 640+ items warlords crafted, 620 alchemy trinket, 3 random class perks and all other slots 615.
98 -> 3 640+ items warlords crafted, 640 ring 1 & 2 (depending on class / spec), 620 alchemy trinket, all 4 class perks and all other slots 615.
And for "special fun" you can use old legendaries which get scaled up too. And to optimize every slot below 590 you can use whatever you want with your perfect 2nd stats. Lets say you want mastery / crit and your 570 gloves have haste / crit on them. you can use level 60 gloves and so on with mastery / crit cause of their scaling up to 590. So you profit more from the 2nd stats. And to min-max you can look up items with your preferred 2nd stats and sockets.
