I try to keep the number of partitions as small as possible. Every partition takes precious system memory (buffers). If you later on find out that you can't fit everything to your music:-partition, you need to backup all data, resize partitions, format, and restore data (or store your music to some other partition, which kind of beats the purpose of different partitions for different types of data anyway...).
I rather make as large partitions as possible and create directories for different type of data. Then the free capacity stays available for whatever data I want to use, and no free storage is "lost" storing some particular type of data. Moving data around is just renaming it.
So with single HDD I'd have two partitions, system and the other for everything else.
The only downside I can think of is that if you're using FFS, validating the partition can take a long time. Personally I use PFS3 for all partitions. Others have found SFS good, aswell.
With WB 3.0/3.1 you want to keep single partition size <=2GB, or some applications will get confused.
