Failover Cluster - Add Witness Disk? I'm trying to build SQL Server 2012 Cluster on top of Windows Cluster 2008R2. 

Can anyone please suggest.. do we need quorum disk in setting up SQL cluster 2012 on top of windows cluster 2008r2
The choice on whether or not to go with an additional non-node vote (I don't want to say "witness disk" here, because an additional vote can conversely come from a file share as well with the node and file share majority quorum model) depends on how many cluster nodes you have.

If an even number of nodes, then add your additional non-node vote (whether it's a witness disk or a file share).  But if it's an odd number of nodes then you will gain no benefit out of a witness disk or file share voter.

Take a look at this TechNet reference on quorum model recommendations.

It is also worth noting that with the entrance of Windows Server 2012 R2, the capability of a dynamic witness has been introduced.  I realize you aren't using this version of Windows Server, but for the sake of a complete answer, here is an excerpt explaining this new feature in Windows Server 2012 R2.  This dynamic witness would be the exception to my answer:


  In Windows Server 2012 R2, if the cluster is configured to use dynamic quorum (the default), the witness vote is also dynamically adjusted based on the number of voting nodes in current cluster membership. If there are an odd number of votes, the quorum witness does not have a vote. If there is an even number of votes, the quorum witness has a vote.