SquishyFish,
I don't think many people consider DT's a 'chemical' ? And like baraccuda said, it's not good to add supplements for things you aren't testing for.
I think it all comes down to the methodology you choose to use. Some will use chemicals on a as needed basis, only supplementing those things that are depleted (takes a lot more understanding and time). Some just do their monthly (or so) water changes and add calcium/alk buffer (which many methods of calcium addition also will raise alkalinity as far as I understand). Then there are the people who use 2 part (which seems to be a good percentage) where you don't have to necessarily test each particular element, but monitor your levels of alk, calcium and pH regularly in addition to frequent water changes and you will keep a relatively good ionic balance.
And these are only a few of the methods. There are countless combinations. It seems easiest however, to start with something basic until you understand the relationships within the system. Since some ions can combine with others in an equilibrium, if you add too much of that one it will use up all of that counterpart, depleting the element. This, as far as I've been able to research thus far, is why it is usually recommended to add both calcium and magnesium supplements when using the liquid supplements. Those forms of calcium will use up magnesium by creating a bond, making both unusable if too much calcium is added without a balance in magnesium.
Anyway, take any regiment of additions slow. Even add only half the recommended dose first. Then check your levels, see what's going on before continuing with more. If it all goes awry, do a water change or two more frequently than normal; balance should be restored.
This is probably one of the most complicated aspects of the whole reef keeping experience (other than expense) because there are so many options, and there isn't an easy way to understand it all.
Good luck!