If you're following the plan to the letter then no, you're not supposed to carry them over. The plan says for us to aim to have between 5 and 15 per day.
However, some people have been known to use their Syns weekly, rather than daily. By this I mean counting their Syns out of 105 throughout the week, rather than out of 15 each day. For example, one day you might have used 3 Syns, but then the next day you've used 25. That would be 25/105 and you'd have 80 Syns left for the rest of the week. Personally this method doesn't work for me, as I need to follow the plan pretty strictly or else I'll just binge.
The only thing I would suggest is to make sure you're at least having some Syns every day. Once you get closer to your target your weight loss might slow down, and one way of encouraging your weight loss to start again is to gradually lower your Syns. Hence you need to start off having Syns so that you have something to lower if you find yourself in this situation.
Hope I haven't rambled too much and that this makes some sense. Good luck xx