If it's a quick scratch a few times a day, then it's quite normal. The signs of excessive scratching are more along the lines of 10 seconds or more spent instensely scratching, jumping up and scratching instensely when previously fast asleep, and biting at the skin.
Routine bathing - once every 1-3 months, can help keep skin clean and healthy and prevent some skin irritation.
The Xeno brand is along the right lines for treating mites, but not the spray and not weekly. Xeno 450 (or Xeno 50 for pigs under 800g) is a topical "spot on" application, used to treat an active mite problem (2-3 doses/applications, around 10 days apart). Some people also use it once every three months to routinely wipe out any possible mite problem.
