Keeping your app up to date if you have any significant quantity of third party gems, is really nice. Meaning, if you need to upgrade, it's easier to stay on top of it than it is to pull an app from say, 4.2 to 6.0. This becomes particularly problematic a few years down the road, for security updates.
That's the other concern, right now all the gems play nice together without issue. It's possible that some of the older gems are supported under 5.x and haven't made the required changes for 6.0.
