My router mostly lives in a router table. I noticed that it was getting less smooth winding up and down, so decided to investigate the insides, using Ray Girling's excellent guide to dismantling. I was astounded by how much caked on dust residue there was infesting the cog wheels - and it didn't just brush off, it needed a wire brush to remove it from the metal gears. It's not obvious how it gets in, but it sure does. Although it doesn't seem to have damaged the metal gear cogs, the orange plastic wheel in the handle gets worn - I had replaced this once before.

If you are experiencening less than smooth winding, worth a clean.