But only if you are trying to raise the alk of the new water to bring it to an acceptable level. Trying to add extra, so as to raise your tank alk this way could very likely cause a precipitation effect in the new water. I'm sure this is not not Bradleyj meant, but I wanted to just clarify in case anyone reading this thought this way.
BTW Joe, I'm with you on the Magnesium. I found (in this tank and my previous tank) with Mg at 1100ppm or so, I could NEVER get my Ca above 360ppm.