For my home, I bought Pottery Barn sofas nearly 8 years ago with cream slip covers - I wash them once or twice a year - they work brilliantly. If in doubt, by a second set of slipcovers (although this can prove almost as pricey as a new sofa sometimes).

Issues are to make sure the slipcovers were washed BEFORE the fabric was made into covers--so they won't shrink when you go to wash them yourself...
also -- if you are having them made consider using a SunBrella fabric that is much more resistant to staining and rubbing than a general upholstery fabric...

Nearly 50% of the slipcovers we make for our customers are in either white denim or canvas. These slipcovers go into very active homes with children and pets. Our customers love them because they can bleach them. Slipcovers made from good quality fabric will last for years and wash up beautifully. Keep in mind that natural fiber fabrics will always tighten up when you clean them - no matter how many times you do it. So "pre-shrinking" would only help if you're using a loosely woven, poor quality fabric. We add a little ease to our slipcovers to allow for the initial tightening. http://www.needleandshears.com/