There are many different answers to this question depending on your priorities and specific circumstances.

What kind of slipcover are you considering?

• One-size-fits-all slipcovers are affordable, but nobody is going to compliment you on how nice your sofa looks.
• Custom slipcovers can be extremely costly. If cleaning is a priority, you need to be sure that the fabric selected is highly stain resistant and cleanable.

Some new sofas include removable slipcovers. This gives you better cleaning options.

• Slipcovers that come with new sofas generally fit loosely.
• Loose fitting slipcovers can be made more quickly by sewers who are less skilled.
• Tight fitting, tailored slipcovers (that do not look like slipcovers) require more skill and time to make.

The slipcovered sofa shown above from Crate & Barrel 

shows the typical loose fitting, wrinkled look. Underneath the slipcover is a thin muslin fabric.
New couches featuring tightly tailored pre-made slipcovers are rare, but they do exist.

The slipcovered sofa shown above is by Simplicity Sofas

Unlike most slipcovered sofas, this is fully upholstered beneath the tightly tailored slipcover and can be used with or without the slipcover.
The best method for cleaning slipcovers will vary depending upon your preferences and the type of fabric used for the slipcover.

• Many slipcovers are made using 100% cotton duck fabrics. Cotton is an ideal material for slipcovers as it stretches better than polyesters and other synthetics.
• Cotton fabrics can be washed on gentle cycle, however it is important to remember to avoid heat. Hot water and hot (or warm) dryers will shrink many slipcover fabrics. Pre-shrunk fabrics are often used because of this.
• Many slipcover sofa manufacturers recommend dry cleaning for their slipcovers. This avoids the shrinkage problem and is also less likely to weaken seams over time.

Before trying to remove any stain from a couch (or slipcover) I recommend that you do a Google search for the exact type of the stain and the type of the fabric. (e.g. How do I get a mustard stain out of my cotton fabric?)

• You will find multiple articles on almost every possible combination of stain and fabric.
• Slipcovers often have separate covers for each cushion. These are designed so that they can be easily removed for cleaning.
• Cushion covers for normal (non-slipcovered) mass produced sofas are usually not removable for cleaning, even when they have zippers.
  • Cushion cores are often stuffed into covers at the factory by huge pneumatic machinery. Removing the covers is difficult.
  • Replacing sofa covers after cleaning is almost impossible without the services of a professional upholsterer.