As long as your other minerals are in range (see Palmer for that info), the only thing that really matters for 90% of beer styles is the sulfate to chloride ratio. Sulfate tends to accentuate hoppiness, chloride tends to make for a smoother maltiness. In almost all cases, there's no need to mess with Magnesium or Sodium at all.
Since my water is calcium deficient but otherwise fine for all-purpose brewing, I simply add gypsum (for my pale ales/IPAs/other hop-forward beers) or calcium chloride (for everything else) until I have plenty of calcium for the yeast to be happy. 5.2 takes care of the pH buffering for me, so I don't have to worry about using salts or acids to achieve the same (in other words, for me, brewing salts are simply used for flavor, not mash efficiency).
For meads, I'll add some calcium carbonate to buffer the pH.
The Brewing Network's Brew Strong show with Zainasheff and Palmer is doing a 4-part water show right now (parts 1-3 should be available for download) that provides quite a bit of info on the topic - if you have the time, give it a listen.
The Fiesty(sic) Goat Brewery est. 2007 & Clusterfuggle Experimental Ales est. 2009
Planned: Fat Man Porter, sLambic II, Brettennial Falcon IPA, Flanders Red, Orange Blossom Mead
Primary: Winexpert Riesling Ice Wine, sLambic I
Secondary: Flanders Red
Kegged:Black or Blue EyePA, Cherrywood-aged Crystal Stout,
2013 dump volume: ~2 gallons