Hi Jason,
You are correct the version 5 products for the complete MailEnable suite will work with Plesk 10. MailEnable does test every major release of Plesk and the integration of the supported MailEnable features within Plesk with each release. Usually all versions of MailEnable will work with Plesk and most times we do not find any problems as the feature set that Plesk supports and the way that these features connect with Plesk rarely (if at all) change. In the instances that we do find an issue the devlelopers here and Plesk work to resolve any problems quite quickly.
The only issue you may have that we have encountered at MailEnable support is that the Plesk support technicians will not help you with any problems unless you downgrade to the version supported by Plesk at the time. This is never helpful but a policy that will probably not change. Not sure why, but probably due to Plesk integrating with so many products it is hard for them to keep track or maybe as their support is outsourced this policy is hard to waver from.