I don't think it would matter that much other than for the actor writing, easier if (s)he doesn't have to think about reverse-writing. The flipping effect is a one-click effect and wouldn't require much in terms of hardware, software or brain-power.
What editing software are you using - the effect should be easy to find in any way. It might not even be an effect as such but rather a transform feature but it would help if the editor was brought out.
I doubt that your editor DOESN'T have this effect but in the strange case where it doesn't, you can also try setting the width of the image to -100%, that works on some editors. But I'm pretty sure there is a flip.horizontal effect somewhere.