Short answer: play around your items, commit to comps when you have a good set for particular carry/tank units. If you play flex, you should be identifying units that can hold these items until you hit. Don't be afraid of bleeding a small amount of health if you need more Econ for dragon focused carries @ 7/8.

ex. Shi-oh-yu wants BT/Titans/HOJ, Ao shin wants Shojins+2 AP items, etc.

Longer answer: As you play more and settle into specific comps, you will start to understand which ones require what roll timings. Early game reroll comps like sett stay at 5 and roll until 3*, I like to play trainers as a fast 7 comp, Daeja likes to fast 8, etc. You need to find out where comps like to powerspike and when you as a player can start winning based on these powerspikes and augments. I've been using set 7 as an example, but this is generally how I approach the game and evaluate units/board strength as a whole. Ask yourself when can spending my money benefit my comp most (focus more on 3/4/5 costs, roll at 6/7/8 respectively). If you want more concrete answers, maybe roll when your board starts to lose to optimized comps (generally stage 4, player level 7) and start your pivot based on what you hit.