You should have a dedicated domain account that is used as a default content access account. This is important since this account is used by the crawler in any of the servers in your farm that has been assigned as crawl servers.
Default content access account needs to have Full Read-permissions to the SharePoint web application you are crawling.
1. Open SharePoint 2010 Central Administration
2. Click Manage web applications under Application Management-section
3. Select your web application's row
4. Click User Policy from the ribbon
5. Click Add Users
6. Click Next
7. Input your default content access account to Users-box
8. Select Full Read under Permissions
9. Click Finish
