You can use additives if you'd like, but they're snake oils. I've bought and used every possibile additive for my setup and have seen no positive effects.
These nutrient companies are looking for more revenue so they'll sell ya anything.
(On a good note, I didn't see any negative effects either.)
Well, thats not all true. I did try Superbud and while insanely expensive, it did shorten the stretch of my plants during flowering. A bit of research into the product and I found that they used a specific plant hormone in their "secret reciepe". You could make hundreds of gallons of your own Superbud for less then $15. If anyone is interested, I will go into more detail.