iTunes Match and Photostream do not use any of your free/paid for iCloud storage. The two main uses for this type of storage are for back ups or for syncing documents because as you say Mail, contacts, calendars etc, don't use all that much space at all.
I Find that document syncing is best used for working documents rather than the storage of historic documents (which are best stored on-line using something like DropBox), so unless you have a huge amout of working documents you will probably find that 5 GB is more than enough.
If you back up one device using iCloud, you may or may not need more than 5 GB, if you back up two or more, you probably will need more, however Being able to back up to the cloud can be very useful, especially if you don't have access to a computer or have infrequent access to one, however unless you specifically need to use iCloud for back up, you will find backing up to iTunes significantly more convenient and possibly more reliable.
I use around 1 GB of my 5 GB.