I'd like to add a different view. My preference is to add all brewing salts to the HLT and get it to the proper temperature before draining the correct volume to the mash tun so it hits the correct temp immediately following dough-in. There is one exception to this rule, however, and that is for calcium carbonate (aka chalk) that is poorly soluble in water unless it is quite acidic. So if chalk is needed, I add that component immediately after dough-in, and stir it for a few minutes to ensure it is fully dissolved.
The problem with CaCO3 and CaSO4 (gypsum) is both of them are less soluble as the temperature increases (which is the opposite of what you'd normally expect). But in almost every case, the solubility of gypsum is still more than enough at strike temp, while chalk very rarely is. All other common brewing salt present very little worries with solubility.