You don't have to create a partition but if you create a bootcamp partition then you tell Parallels to use that partition instead of installing Windows again. If you're planning to play games or do demanding tasks in Windows I would suggest creating a bootcamp partition. That way you can boot into Windows for demanding tasks (eg. gaming) or just use Parallels when you're not doing demanding tasks.
MacBook Pro with Retina Display | 15.4" 2880x1800 | Core i7-3615QM @ 2.3GHz | 8GB DDR3 1600MHz | GT 650M 1GB GDDR5 | 256GB SSD | OSX 10.8.5 | Windows 8 Pro