How often to refill/refresh pens?

Two of my favorite inks—Diamine Imperial Purple and Birmingham Pens Electron—had recently seemed very dark and dull to the point of looking almost black despite decently heavy use of the pens they're in. I refilled a those pens today, largely because I was bored, and I noticed that immediately after their writing seemed much more vibrant.

So now I'm wondering if it's a good idea to refill or refresh pens when they aren't nearly depleted, just to keep the ink looking good. To that point, it's it better to top off or do a totally new fill? And relatedly, is the ink that's been left sitting in a pen somehow deteriorated in quality such that I should avoid putting it back in the bottle during a refill?