I was always told not to carry Syns over, mainly because if you do save for say a takeaway and drinks on a night out, chances are you will carry on eating lots of Syns the next day and end up having more Syns then your allowance.
My MIL carries her Syns over so she can have a few drinks on a Saturday night, but she really struggles to keep within her Syn allowance.
It's whatever works for you really x