You can, but I would suggest that you use at least 5 a day anyway.
If you look on the website there is a section all about Flexi-syns.
Flexi-Syns - say you are going out for a meal/drinks. So you pick a maximum limit to of Syns stick to for the night, and once you hit that limit you stop having anything with Syns in. And then get back in plan as normal the next day, using your 5-15 Syns a day.
(Does that make sense?)
The other way to look at it is the you've got max 15 Syns a day, which makes 105 Syns a week. You can count them weekly and use them as you see fit. I wouldn't suggest avoiding using any Syns at all on some days though, use 5 a day min.
The people that don't use them don't have the best losses.
Does that help/make sense?