As we age, it is natural for the toenails to curve more. This happens because the natural support to the toenail wears away and the toenails folds around the bone beneath it. While this does not happen to everyone, it is very common. In these cases, trimming the toenail straight across will result in sharp corners of the toenail digging into the skin. People whose toenails grow in this fashion would be better served by slightly rounding the corners of the toenail, either when they are being trimmed or using a file to do so afterwards. This will relieve the pressure on the skin and keep the toes comfortable.
A word of caution, however. Do not mistake "slightly rounding the corners" to mean "dig out the sides of the toenail." If the toe is painful, whether before or after you trim the toenail, stop what you are doing and get it properly evaluated and treated by your foot doctor in Houston, TX. I guarantee it will be less painful than doing it yourself!