For rolling upgrade purposes yes. But only to a Logical standby. And you can also use the transient Logical standby rolling upgrade as well if you are a Physical standby user.
See the following:
Mixed Oracle Version support with Data Guard Redo Transport Services (Doc ID 785347.1) on Oracle Support
MAA Rolling Upgrade paper at http://www.oracle.com/technology/deploy/availability/maa/maa_wp_11g_upgrades_made_easy.pdf
Transient Logical Standby Rolling Upgrade MAA paper at http://www.oracle.com/technology/deploy/availability/pdf/maa_wp_11g_transientlogicalrollingupgrade.pdf