We have white slip covers on the sofa. The covers do need to washed from time to time. I had not washed them for a while (maybe for a year), it is bit of a hassle to take the covers off and put them back on. But surprisingly I do not think those have needed washing until now, maybe my standards of cleanliness or whiteness are not that high.
Removing the covers for washing reminds me of why I made the slip covers in the first place. The sofa frame and the cushion fillings are in very good shape, but the upholstery is really worn. The pillows have holes and stains. Considering it's age, about 30 years, it's a miracle the upholstery is the only thing that is worn. The sofa is obviously well made, I love the it, the proportions are excellent and it's good to sit on. Odd to see the sofa in it's original colour.