Any saddle that is well-fitted to the horse should be able to be ridden in without a saddle pad and not hurt the horse. It doesn't matter what kind of saddle, or what kind of flocking it has, or how much money you spend on it. If it fits well, it won't hurt your horse without a pad. However, it is a good idea to use a pad to protect your saddle from dirt and sweat. Keeping your saddle clean, and conditioning it regularly can prolong the life of the saddle.
Choosing the right saddle pad can be important in keeping your horse comfortable. Sometimes using a saddle pad can make your horse uncomfortable. If you use a pad without wither clearance, it can rub your horse's withers. If your saddle pad slips or bunches up, it can create pressure points and make your horse uncomfortable. If you use a lot of padding under a saddle that fits, the saddle can become too narrow over all the extra padding and actually pinch the horse and/or restrict his movements. Too much padding can also upset the balance of your saddle or make your saddle more likely to slip. However, corrective padding does have its place. Shimmable pads are great for horse's with oddly-shaped backs or uneven muscling as the shims can fill in areas that need filled without custom fitting the saddle. Padding can also be used to absorb excess shock on saddles that have hard panels.
Use a plain, quilted, cotton all purpose pad with a bit of contour for wither relief. My saddle has been fitted by a professional saddle fitter and it is relatively new so the panels are soft. I only use a pad to protect the saddle, and saddle pads are fun! It's a cheap way to get colourful! But I have ridden without a saddle pad before. Saddle fitters recommend riding without a pad when trying saddles because you can see sweat patterns from the saddle more clearly, and evaluate the saddle fit without distortion from a pad.
My husband uses a thinline pad. He has an older stubben with harder-packed wool flocking. His saddle was evaluated by a fitter and the panels are the right shape for his horse's back. They're just not as soft as they could be, so we use a thin line to absorb some extra shock. We do not use a bulkier pad for shock absorption because the saddle fits really well without added padding.