Safety net is only useful if you are unsure about some units or are doing equally well in all of them. It really depends on what you are doing and your abilities with them/stress levels/study and organisation habits.
I had 12units in year 11:
*Eng (Adv)
*Maths (2u)
*Legal Studies
*IPT
*French (Cont.)
*Italian (Beg.)
Maths was my worst subject, I hated it and had to study harder than anything else to keep up, in the end it would have been counter productive to keep it. The time I saved was good to give me some time to relax and calm down, plus I could spend more time working on my other subjects and while the french exam was terrible and I did worse than expected I doubt my maths marks would have been better. My school infact encouraged 10u over 12u.
Its up to you but don't hold on to the 2u for the sake of it, unless you REALLY think it will make a difference, or you're doing fanastically in all units and can't chose what to drop why do it, after all you will be competing with those who are only doing 10u.
Best of luck