I too am not an expert here, but if your piece is pre cut to size you don't want to wash it. I have been reading up on this to wash or not to wash and have gotten different answers from diffrerent people. The rule of thumb seems to be if you start with washed fabrics stay with washed or visa versa. I would be afraid that washing would not only shrink it beyond the size needed, but I would also be afraid of distortion. No matter how careful we are in our washing and ironing, we end up with some distortion. So if it is cut to size I wouldn't risk it.
The census is equally divided about wash or not to wash and lots of ladies are going with the not. As long as you don't mix the two. Either not or wash, but never one of each. So there is my half cent, hope this helps.
Rita