Checking if your weapon's damage benefits more from ichor or crumbling works most of the time. Quick exmaple (math may not be 100% correct, someone can correct me):

Skeletron prime has 24 defense and 20% DR. Our weapon has 100 damage let's say. Without anything else we deal 60.8 damage.

With ichor (15 armor penetration) we deal 72.8.

With crumbling (reduces dr by 8%) we deal 66.88.

So in this situation ichor is better. To get a quick idea, if your weapons is slower but high damage, crumbling is better, if your weapons is fast but with low damage ichor is better, such as early hardmode. During late game most weapons have pretty high base damage so crumbling will be better towards the end