it won't make the right side of the fabric look any different; you'll see the strands on the reverse side though, just like it looks pre-felted.
One thing to watch out for when your felting is to make sure it felts evenly; if your stranding is tighter or looser in some places, it will affect the shape. I made the felted bag from S&B a couple times, and I experimented with fair asile/stranding... instead of having a square bag it ended up kind of round and bulgy in spots
Best thing you can do is keep a close eye on it while it's felting... if you see some parts shrinking in more than others you can stretch them out while it's still wet, and it will be okay.
HTH