Should I put flux on the tip of my soldering iron?

No — flux belongs on the joint you're soldering, not smeared onto the iron tip itself. Putting flux paste directly on the tip is a common shortcut that causes more problems than it solves: flux residue baked onto a hot tip over and over builds up as a hard, dark crust that insulates the tip from the joint, makes heat transfer worse, and shortens the tip's working life.

The one place a small amount of solder (not flux) belongs on the tip is "tinning" — keeping a thin, shiny coat of melted solder on the tip between joints, which is done by wiping the hot tip on a damp sponge or brass wool and then touching it briefly to fresh solder. That's a different step from applying flux, and it protects the tip rather than fouling it.

If a joint needs extra flux because it's oxidised or dull, apply it to the joint itself with a flux pen or brush, then bring the iron to the joint as normal. See how often should I replace a soldering iron tip for the rest of good tip-care habit.