As Nickie said, technically no, but quite a lot of people do, with success.
The only problem with that as I see it, is that if you use all your syns up early on, then you're more likely to bend the rules towards the end of the week.
If you're saving them because you simply haven't used them, then as long as you are having a min of 5 a day, then maybe you don't need your full 15. They do say between 5 and 15, rather than a staunch figure to stick to.
Don't forget you can flexi-syn for special occasions too.