It SHOULD be mandatory for painting over a Semi-gloss wall!!

Sanding a Satin wall is optional.

Washing a wall is more important IMO!!

* If it's a more moderate knock-down, sanding can smooth-off any hideous sharp edges.
* Also, even if you have orange-peel, sanding can knock-off any bigger clumps, etc.
* Sanding BETWEEN coats?!?! Rarely, unless your wall is smooth; OR....you're doing "eye-critical" work like trims, cabinets, doors, etc.

Faron