It would be useful if your plant was suffering from magnesium deficiency, which does happen. But it's not going to flush any salts / minerals away; it is, itself, a salt (magnesium sulfate). Even if your plant did have a magnesium deficiency, it would take very little magnesium to correct it, and is hardly something you'd need to do with every watering.

There are a lot of people who use epsom's salts as an additive to their fertilizer. It's not that uncommon, nor is it unhealthy for the plant if used in moderation and the right proportions.

I have a recipe for a fert that calls for epsoms salts which has been discussed to death on epifourms, everyone it seems who uses it has had good success with it, and I've used it myself.

I agree it does not flush salts out of soil, that should be done with plain water, not with anything added to it, but it does have it's perks.

I was under the impression that most fertilizer formulations now include magnesium and other micronutrients as part of the deal, and I know mine does (some Miracle Gro thing; couldn't tell you the specific product 'cause the packaging was gone a long time ago). Adding magnesium to a plant that already has it wouldn't help, and we're all in agreement that it's not going to flush salts out (we are, right?). That said, there are plants that have a higher-than-average need for magnesium, which could benefit, but without knowing how much to use (how much do you use, mentha?) and whether your plant actually needs it, it seems safer to me to err on the side of caution. There's also a complicating factor in that city water supplies can vary a lot in how much magnesium they contain before you add anything at all. But I'm willing to retract my above post if anybody has specific information about . . . well, about any of this.

Mg is found in plant tissues in the range of 5-15% of the amount of N. Another way of saying it is there should be 7-20 times more N available for plant uptake than Mg.

Al

Sorry I was on vacation, but I use epsoms salts for plants like Schlumbergera where a magnesium deficiency will kill the plant in short order. I also use it for tomatoes, peppers, and other nightshades. I also use it whenver I find plants with yellowing leaves.

I don't recommend it for everyday use, but like I said in the above post in moderation it is safe. I don't use commercial fertilizers with the exception of Eleanor's VF-11. I make my own fertilizer from a recipe picked up online being sold for cactus, it works well for any plant except the slowest growing plants like Sans, which too much can cause striations of the leaves.