Ran into an issue when running SQL 2008 R2 Reporting Services in Sharepoint 2010 Integrated Mode. Only those with Full Control permissions could see the Subscribe option in the Actions Menu. However, users could Manage Subscriptions from the Item drop-down menu in the Report library.
By default, the OOB Permissions Levels do not have the Create Alerts option set. Here’s a way to create a Permission Level that will allow you to do that.
Go to Site Actions > Site Settings > Site Permissions > Permissions Levels > Add a Permission Level
In this case; I named it Report Permissions with the description of Create Alerts, Subscribe for Reporting Services.
Under List Permissions; Click these Options:
View Application Pages
Under Site Permissions; Click these Options:
Browser User Information
Use Remote Interfaces
Edit Personal User Information
Assign this Permission Level to your Viewers Group(s).