The SuDe is only a guideline. Some products like Dolls, ice cream, and motorcycles have a big difference between what they can sell in good seasons and what they can sell in bad seasons. The SuDe is a good guideline in good seasons. So in the winter dolls sell the best and as long as the SuDe is not too far above 100% you should be able to sell the full amount. The problem is the demand in the other three seasons is much less. You could have trouble selling the full amount when SuDe shows 50%.
So to finnaly answer your question of "When should I lower my selling price?" Lower it when you can't sell all your goods ;p
Well, I just lower the price until I sell as much as I wish. It's too much trouble to set the price for every season. Usually I'll under produce dolls and set the price a little high. When my warehouse gets too many dolls in it, I lower the price.
I think everybody sets their prices for these types of products to satisfy the market on low demand seasons. Then in high demand seasons you just sell for less than you could if you were online 24/7 
Alternativly just show the Average Demand always. Now Only Seasons with Demand = 0 are handled specially.
If Demand is 3 - 36 - 14 - 0 the Average Amount of 13 should be shown in Prod. Log. Or alternativly (but much better, i suppose) - a pricesetting for different seasons
