Partitioning HD's and Multiple OS's

I never really partition my hard drives. Is it smarter to reserve a smaller partition to install the OS on and have a separate partition for data? I usually do one full partition and just install the OS and go.

I ask because I just ordered two 1TB HD's for dual booting, one for Windows 7, and one for Snow Leopard. I am thinking of Triple booting as I also have a 320GB HD that I could use for Ubuntu. Would it be smarter to partition the 320GB hard drive into 3 partitions (1 for each OS) and just use the two other HD's for data?