I follow Griffin’s advice.
You may find this reference helpful as well.
Also, When it comes to Obsidian affix, it won’t roll a duplicate affix. So if you want obsidian Crit damage affix but your item already has that affix you won’t get that option on the roll. I use this fact to my advantage by adding rare/epic affix (cheap) on the item for affixes I don’t want obsidian to roll. For example I may add hp or mp regen rare affix, and then roll obsidian, so that obsidian roll doesn’t include those 2 afiixes. I only mention this because once you roll obsidian u can’t remove it unless u remove all other affixes as well. Hence the difficulty in crafting perfectly affixed gear 
Here’s a list of affixes.
Hope these resources help as a meager supplement to Griffin’s recommendation 