buymeasoda saidrotate your mattress every month and please change your mattress at least every 10 years
You can't flip many newer US mattresses, they don't have fabric quilting on the bottom, and of course many are pillow tops. Even rotating is being discouraged by some manufacturers. And our last innerspring was so heavy, a king with a very high spring count and very thick, it would take several people to do it, my husband & I really couldn't manage it alone.
And our new Sleep Number air mattress can't be flipped or rotated, either, but hopefully won't need to be, since it has no springs to sag. It's got a slight pillow top, and the air hoses enter at one end, that you want to be at the head of the bed. But it means we've gotta keep it clean and fresh. So we put a washable, breathable, but also nearly water impermeable full cover on it. In case we spill something we're drinking while watching TV. And then of course if you're not careful with the lube...
Another reason for a good mattress cover or pad is that the human body is always sloughing off dead skin cells, and they can pass through ordinary bed sheets, collecting beneath. Likewise skin oils & sweat can penetrate sheets to some degree, and especially if you sleep naked, or are having a nice playful romp with another guy.