I've always had 15 until this year. The only time I cut down was the last couple of weeks of approaching target when I reduced to 5, never less.
This year my consultant has been recommending 10 syns to all of us. She does say though, that 5 - 15 syns is allowed and it's not a problem if you go up to 15, they just recommend 10.
Lets face it, 5 syns = 100 cals = 700 cals a week. You need }+/-3500 cals to gain/lose a pound so it's not going to make that much difference as long as you do everything else correctly!!
I think the 5 syns deficit makes up for those "little bits" many people forget to count!!