In mass effect 2 is is it absolutely necessary to have full paragon/ renegade for no one left behind?

Without having nearly full one or the other, you won't be able to resolve two arguments that will show up once you complete the participants' loyalty missions. Tali and Legion, and Miranda and Jack will each get into a fight, and without a lot of Paragon/Renegade you will have to choose one side in each of the fights, meaning that the other person will loose loyalty. You can still have the unloyal person survive, but you may have to try the final mission several times.