I think the best way to prevent 'ambient dust or grease' from entering a kettle would be to put the lid on it.
The 'deposit' forms because your water contains quantities of calcium ions (or possibly magnesium). The solubility of such depends strongly on temperature, and reaches nil at boiling point, when they are precipitated out of solution in the form of calcium carbonate, i.e. chalk.
If you store the kettle with water inside, it will slowly evaporate and probably cause a few more ions to precipitate over the course of the day. But compared to what happens when you boil the thing, this will be negligible. In other words you're screwed either way. Mr. Bear is propagating an old wives' tale...
If you can run to the expense of a water filter or softener, this will help significantly. However, there is one very easy way of removing the chalky deposit, if you happen to dislike the prospect of stalagmites in your kettle: Vinegar. Cheapest vinegar you can find in the store, one part to ten parts water, fill the kettle and let it stand overnight, next morning the chalk will just wipe away. If not, repeat with one part to five parts water. Don't forget to rinse.