Mongo - Sure. I'm hardpressed to find genuine reasons to use MongoDB these days. Postgres provides everything I need in a persistent data store and has a ton of other stuff I didn't know I'd need (till you do!). Mongo was great to use with something like node.js where the async/schema-less model translates perfectly but using Postgres with the async driver[1] isn't bad either. Add in hstore, JSON support, plV8 ... and what exactly would somebody use Mongo for? :D
Redis - No clue what you're saying here. If you're using Redis then you're keeping your data in memory and dealing with low level structures. It's for completely different use cases. A classic example is rate limiting for an API. Doing it in Postgres with a disk I/O per request would cripple any semi-popular API. The data doesn't need to be exactly persistent (if we miss a few updates due to the server crashing we don't really care). In exchange for that it's blazing fast for individual writes that we can batch together to backup to something like Postgres (or Redis's built in persistence like AOF).