Mandatory enchantments MUST be revealed when triggered. You can not delay them. However, you don't have to pay for them. Even if their reveal cost is 0, you can choose to not pay for them and let them fizzle, getting no benefit out of them. So, if you had both Block and Eye for an Eye on the same creature, you could choose to let block be destroyed without an effect in order to benefit from the Eye for an Eye on the first attack.
It's also important to note that with all of the expansions that have come out recently, a lot of mandatory enchantments have different triggers now days. For example, Glancing Blow is triggered at the beginning of the Roll Dice Step. Which makes it good for stacking with things like Block, because if the Block works, then you don't waste the Glancing Blow.
