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.
SW position on this has varied over the years but current advice is that it is 'not recommended'. That said lots of people do it successfully. If it helps you stay on plan and you don't feel deprived on the days when you are having fewer syns then why not.
My consultant said this week that it is 'frowned upon' as you may feel deprived... that said if it works for you then go for it. A girl at group has lost 3lbs whilst on holiday and said she had saved her syns throughout the week so she could drink at the weekend. Seems to be working for her!
I never usually eat any syns during the week as I'm content with free food, I then have a few pints on a weekend with no problem. It may be frowned upon sometimes but I've lost a 15.5lbs in my 6 weeks of slimming so I think it's perfectly fine!
I used to do this lots when I first did SW years ago; meant I could have a 'normal' night out with friends every now and then without either just blowing all the good work or feeling exiled to salads and diet coke! I never starved myself to do it either, stuffed myself silly with free foods for the rest of the week so I could do it.
That said, I do think it depends on the individual and where you're at; I know that 15 years later this wouldn't work for me at all as I would prefer the odd glass of wine or a bit of extra cheese more frequently than once a week, plus I think my body needs a bit more love than it used to : ) so really depends on how it works for you I guess.
I have lost 10lbs in four weeks I have lost more the weeks I saved mine for the weekend than the weeks I ate every day. I had my syns over Saturday and Sunday last two weeks and none either side and I have lost 1/2 a stone. Its working for me and I do not feel deprived, I feel I am working towards a treat. If it stops working I will stop.