I reckon u should keep the extra units...most ppl waste their frees anyway! Don't be discouraged if you think your assessment marks in one subject are letting you down (unless its extraordinarily bad), keep it. The extra units should be seen as an emergency exit plan if there is a bad exam or wateva.
E.g Modern history is one of my best subjects....in the exam my entire class surprisingly bombed. Most people were relying on this as their strong subject, and their uai suffered. As a 13 unit person it didn't effect me.
If you can do extra units of english it's also recommended, because u know 2 *have* to count, so it just reduces pressure on advanced english. I hated 2u english, with a passion....and no1 can understand why i then chose to do 4u. But that was the best decision i ever made, i did well in the third and fourth units which scaled me well and 1u of adv was disregarded....final result: 99.95. Moral of story: if u can handle extra units without causing a decline in ur existing units, i would imagine its preferable - plus you learn more stuff and become a more well-rounded and educated individual!