It is not necessarily that simple.
If your miners are already paid off, the higher their uptime, the more you will earn if the price of electricity in a given period allows you to make profit.
But it won't always be profitable for you to turn on the oldest, least effective miners.

It is different if you have just invested in a given part of your mine's equipment and you have to pay it back.
Then obviously the uptime should be as close to 100% as possible.
But even 85% would let you pay off the equipment if you had access to electricity at a very favorable price.