I live in a flat with traditional wood flooring boards. They are pretty much in good condition although some have gaps of a few mm. Anyway we want to fill those gaps probably using Bona Gap Master (I heard it's good) and sawdust + resin. My main worry is to stain the floor with a darker colour and screw it all up. What is the procedure to fill gaps and stain the floor?
If you sand it you'll eliminate the charming deep grain and rough-sawn artifacts. I wouldn't sand anything but what seems like an injury hazard. I'd also leave the gaps empty. A hard-floor vacuum will keep it clean enough.
To the second question, staining without sanding will give very dramatic color variation in the open grain and saw kerfs. You'll get a nice rustic look. If you prefer a more modern, uniform color, sand it well to an even smoothness after you fill the gaps.
Having said that, I'm voting to close the question as primarily opinion-based.