Firstly there's no need to register to explore the block chain, registration is only required to use certain features. No personal information is taken from you, and any information you do provide will not be shared with anyone or anything, ever.
Currently the following features require registration:
- Favourite addresses
- Set addresses alias
- Set default currency
- Post and respond to messages on addresses
Feel free to use a throw away email address. The email address you provide will be used to send alerts that you set up and can be configured in the account section.