testing for them, although I personally don't feel its necessary to test everything.
You should watch magnesium, since it greatly affects the Calcium/alkalinity relationship...but unless you have reason to suspect iodine or strontium deficiencies, I wouldn't worry too much. Its the 'fully stocked with coral' tanks that tend to use up traces quickly, and require supplementation...but if you plan to add ANYTHING then you must test for it first.
Iodine is deadly to fish and inverts if over-dosed.
Former advisor and planted tank geek...life's moved on though.