Depending on how your application is written, READ_COMMITTED does make sense when used with Hibernate. Assuming that your DB uses R_C as well, I suspect your app won't have any issues with the cache running with an isolation level of R_C. :-)
REPEATABLE_READ is the cache default as it offers a stronger isolation level for a very small penalty if used in a read-mostly situation.
Thanks for the insight. Just wanted to make sure I wasn't missing anything.