If you're operating out of an airport with a long runway, sea level, no obstacles and so on, your 5 kt tailwind might not do too much for you. But if you're about to takeoff from a short runway, or a high elevation airport, or terrain is a concern, you either have a set of calculations previously done handy or you advise ATC you're not ready to depart and run a new calculation. It takes 30 secs on the FlySmart thingy.
Don't modify the performance numbers so arbitrarily, especially on a high performance aircraft. Too many factors contribute to a takeoff to "just add/subtract 5 kts for this or that"