Keeping the hair shaved and the sores clean applies in every instance.
Each horse is different, so the topicals that work on one horse might not work another.
There's also the risk factor of cellulitis as has been mentioned, if scratches are left un-treated.
There's a huge huge difference in treating the occasional scratches vs. chronic scratches.
Chronic scratches can require big changes in diet and living conditions.
My pasture is very acidic; it's a breeding ground for fungal/bacterial "things".
One horse never gets scratches, two get them occasionally, the fourth is chronic.
Along with diet change, I re-did his stall that included grid mats. He comes in at night, out every day regardless of weather. I change his shavings every 3 - 4 days because his urine can aggravate the scratches.
This horse also has environmental allergies and has oat/corn/soy intolerance, so he's just one big disfunctional immune system
Scratches is the Pandora's Box of equine skin issues - it can range from not much of anything, to causing cellulitis and literally putting the horse's life in jeopardy.
No matter how minor the outbreak, it shouldn't be taken lightly. Besides, it hurts/itches and is uncomfortable for the horse. It can sometimes be the reason for a really sensitive horse's foot-stomping; as I have already learned