purposes of commentary and criticism, it seems to fit squarely within fair use boundaries. One court has already ruled that screenshot reproduction, even for purely commercial purposes, constitutes a fair use.
Getting permission is always better than relying on fair use. If you have permission, there's no need to worry about pesky cease and desist letters or DMCA takedown notices. The downside to asking is that it alerts the owner of your intended use in case for some reason they're opposed to it. Also, before you ask, check the apps end user license agreement (EULA). The EULA may give you a limited right to use screenshots for review purposes. If not, you can email the owner and get permission.
Just saying' dept. Some people have successfully claimed fair use even after a copyright owner refused a request for permission.