I only clean mattresses with specific machines and only slightly dampen some staining if necessary. Rust in the springs is not a good idea. Some of the more expensive mattresses have a layer of memory foam underneath the top cover than will be crumbled away with hot water.
Its mainly critturs and dander rather than dirt that you will deal with. Sometimes you still come across people who dont use a mattress protector at all because they feel its too warming. You will need to do these every week. Once a fortnight is good enough for people who have a mattress topper with a washable cover.
But it is an area where you will be astounded how filthy a lot of people are, esp compared to what they will pay for a carpet they walk on, considering they spend 8 hours a day in bed.
And dont forget to take precautions and not carry anything home. Its easy to pick up demodecs.