With the Eco-Balls would it be OK to use fabric softener?

You can use fabric softener with Eco-Balls.  The manufacturer says: "You should find that the garments are naturally soft without the use of fabric softener.  If you do miss the synthetic smell we suggest that you only use an Eco softener such as Ecover, but do not add it to the wash.  When the wash is complete, remove the Eco-Balls, add the softener and switch the machine on to a short rinse".