I use IO/Reef Crystals mainly because that's what my LFS carries and I don't want to pay to ship buckets of salt. I would like to try Tropic Marin if I ever find it in a store, though.
I usually do most of my water tests before a water change (to see worst case scenario). Mainly concerned with Nitrate, ALK/KH and Calcium since everything else is pretty stable.
I don't always do a test to a new mix... but I would suggest it anytime something changes such as a change in salt brand OR a change in your water supply. I occasionally check PH, ALK/KH or Calcium just to make sure they are close to my tank parameters.