Okay a bit of history. SW used to say that you could not save your syns and that you should use flexi synning for special occasions. Then they changed their position and said that you could, just as long as you didn't mind having fewer syns for the rest of the week. Now they have backpedalled a bit and say that they do not recommend saving syns but they do not categorically state that you cannot.
So no definitive answer to your question I'm afraid. if it works for you and you continue to lose weight at a rate that you are happy with then fair enough I say. It doesn't work for everybody and personally I prefer to stick to daily syns.