The first cull for me is when the sprue plate is opened. Any base that is not filled out gets flicked out before I dump the rest. After that I cull again after they cool but will pull an obviously bad boolit if I see flaws as it rolls around in the cooling area. I keep an eye out for more flaws every time they're handled. I also consider what the boolit will be used for before rejecting very minor flaws. If I know it will be used for plinking I will tolerate a hairline mark as it won't affect the balance of the boolit much for that practice. If it's for a gun I use for competition I try to find all flaws and recycle them. As they're handled to powder coat or lube I watch again for flaws. PC seems to hide hairline flaws so I give them a good look before dropping into the PC tub. They get another glance as they're loaded and if I see a flaw on a loaded round it gets rejected as well. Fortunately those are very rare.
I don't think I pick up too many individual boolits and roll them through my fingers to inspect unless I thought I saw a flaw but I try to be vigilant for bad boolits throughout the process.