The trick to avoid ingrowing toenails is:
When you reduce the length and cut the corners off, Always make sure you can see the points of the cutters peeking out the other side of the corner before you make the cut. In other words, never cut down towards the flesh so that you have to break or pull the bit you are cutting off. Doing this breaks the nail growing under the skin,off at a right angle and makes a spur that later grows into the skin as an ingrown toenail ... ouch.
I actually never file or cut absolutely straight across the nail as I think a gentle curve with soft corners (like a rounded square nail) looks the prettiest, is the healthiest and elderly people should not be left with sharp corners to their nails ... in fact I do not think it is a good idea for anyone to have these sharp corners on their toenails that I am seeing (especially when I see enhanced toes) it isn't even pretty.