While there's no real correct answer for every situation it's a matter of what your requirements are.
Yes you'd want to be on the latest version, because it's usually the most feature packed one and usually the one where bugs in the previous version have been fixed.
No, you don't want to upgrade because a critical (unknown) bug might be present in the latest release. Which might be a big issue. And since you can't (well you can… but it's difficult) downgrade to a previous Mendix version, you'll want to be careful about upgrading.
So: Do you NEED the new features in the new release, or is there a critical bug that's fixed in the new release that would really help you out? Then yes, you'd probably want to upgrade. If not it's usually safe to stay a couple of versions ‘behind’ the latest release. Which is a standard practice in many organisations. an N-2 approach worked well for 7.23 for me.
Also, you're not expected to update. That's always of your own choice. Currently Mendix 6,7 and 8 are supported.
So it is advised to upgrade when you have applications running which have been created in Mendix 5.
Edit, and now i see you're specifically asking for releasing on the appstore. In this case, if it's for the private appstore. Check with your fellow developers and choose a a stable version of mendix to build modules and widgets in, which is old enough to be imported in all the projects you would like to use it for.
Otherwise if you want to release public modules. Pick a stable release in Mendix 7 (Which is probably the version most apps will be on right now). As these can be imported in Mendix 8 as well.
To add to Lars’ great advise: Mendix does not strictly stick to releasing bugfixes: Major DOT Minor DOT Bugfixes. This is only maintained for 7.23 for upcoming 8.6 and for the later planned 8.12. Meaning if a bug is found in one of these versions, it will get fixed in a later bugfix-release. This is not so for all in-between minor-versions. Something to take into consideration.