supplimenting is on a tank by tank basis, it all depends of what is being consumed by the biologic of the tank. if you have a high calcium demand reef (clams sps, with growth) then you will want to suppliment Ca and ALK, as these are in contant demand. water changes help replace trace elements that are very hard test for and only needed in small amounts. these shouldnt be dosed seperately due to lack of control of levels, and the effect it may (or maynot) have on a tank. (when i say tank, i am referiing to the chemistry of the water as well as inhabitants). lower demand tanks (LPS softies) have other issues that have to be addressed on a per tank basis (chemical warfare, nitrates)
there are many products on the market that im my humble opinion are worthless to purchase (i am not referring to any manufacturer, i refer to MOST) they can lead the inexperienced hobbyist to to more problems tha they cure. also the mark up on these items is astonomical (1000's %), and can be made at home for far far less than even the cost of the salt mixes for H2O changes.
as we learn and grow in the hobby we pick up tips and tricks that will save money, time, energy, and frustration.