plussing is based off of the rarity of the drop. This rarity is not the rarity of the item(white, green, blue...). This is shown by the number of stars on the piece. In a few cases, tiering and stating made the number of stars drop. Idk why, and it doesn't make sense. But, the chance to plus is greater with few stars. hammering, or natural od, will always be more rare.
Now alot of the higher lvl gear does not display the stars, but still the item still has a rarity of drop.
The general rule of thumb is as pinkhat stated: plus gear before stating, tiering, hammering, adding runes/slots.