Need to add Domain Controllers to LEM

Hi,

We have a new setup of LEM in our environment and we are in the process of setting it up and adding new servers which need to be monitored via LEM. Here is the info:

1. There are at least 2 domain controllers present at all the sites and we have 3-4 sites.

2. All the Domain controllers except the ones in  the DR site are in Sync( Replication)

Now, the question is:

1. Do we have to add all the domain controllers to LEM or is there any calculation or theory behind it which we can refer to add DCs to LEM?
2. In what cases all the Domain Controllers are required to be added and in what cases all need not to be added to LEM?
3. And at last, which DC should we add to LEM for optimum monitoring?

Waiting for your valuable replies.

Thanks

Devesh