MessageGroup balancing questions

We're considering using MessageGroups and I have a few
questions about it:

1) Is it appropriate to use JSESSIONID for JMSXGroupID
in a web scenario?

2) Will messages without JMSXGroupID still be balanced
in the traditional way?

3) How are new MessageGroups balanced/distributed?
Round-robin, chooses consumer with fewest groups, or
chooses consumer with fewest messages in queue?

Thanks
Mike