That is dependent on yourself...it would also depend on what size of harddrive that is currently installed and what plans you have for later on.

My rule of thumb....
1)anything less than 20gig drive then no extra partitions
2) 30 or 40 gig drives then I would make a 'C' partition of 10 to 20 gigs and use the rest as a single or multiple storage partitions such as "My Documents"with music files and such personal data and maybe a different partition as just plain storage.
3) anything over 40 but less than 160 gig I would split evenly into 4 partitions
4) I do not like large drives as my experience tends to see them over-work themselves and crash quicker than they should.

Just a reminder though ...before doing anything...locate all the needed driver files before reformating and reinstalling so as to have them when ready for them!