In my opinion and experience, saline-filled breast implants should be filled to their rated volume and no more than 10 % more than that (which makes a significantly noticeable difference in size). Over-filling, in my experience, means filling the implant beyond 10% of the rated volume or beyond the manufacturer's limit. Like under-filling, this is a bad idea for palpability and perhaps long term results.
Within the rated to 10% extra volume, I do not see a consistent difference in feel. Most of the feel issues have to do with how much you can feel the saline-filled implant on the side and lower pole of the breast where the pec muscle doesn't cover.
Incidentally, for first time breast augmentation, over 80 % of my patients are still choosing saline-filled implants when I try to objectively list to them the advantages and disadvantages of saline-filled implants compared to silicone gel-filled implants. Unless there is a size asymmetry to adjust for or the patient has a preference for max fill compared to minimum fill, I usually fill them to the middle of the range.