Just be sure the blanket lays flat.
Sometimes the blanket can be cut apart and sewn back together to get a more-or-less even thickness to the filling.
If it's warped/wonky/lumpy/bumpy - it will make the whole quilt the same - and be very difficult to work with.
A friend of mine tried to be "economical" by using a warped, worn out baby blanket in a quilt she was trying to make. In this case, she was being CHEAP - and later told me that it was a mistake to use it. Tried to tell her in a nice way that it wasn't a good idea, but no-o-o, she thought it would work. Sometimes one just has to try it for oneself before becoming convinced. :?