Dashi powder? Use sparingly, if at all
(Update: Since this was posted back in 2007, additive- and MSG-free dashi stock granules have become available. Look for words like "mutenka" (meaning additive-free) or "MSG-free". For example, see this product sold by Japancentre, or this one on Amazon.com.)
I was recently sent a book about Japanese cooking for review. I wasn't too impressed by the book for a variety of reasons, but one thing that really bothered me was that it used dashi stock powder for practically every recipe. (What made it worse is that the book's title proclaimed the recipes therein to be "Healthy".)
Dashi stock powder is akin to soup stock cubes in Western cooking. Like soup stock cubes, they are a very convenient way to add a concentrated dose of umami to a dish. I do have a box of the stuff in my kitchen which I use on occasion.
But keep in mind that dashi stock powder contains quite a lot of MSG. The good or bad of MSG may be a debatable subject, but when it comes to food additives I always like to be on the cautious side. Besides, with the right ingredients making dashi stock from real ingredients, even a vegan version, doesn't take that much time - and tastes a whole lot better too. This is different from the time and effort, not to mention the mess, needed to make a good chicken stock, for example. On my list of Japanese pantry essentials, I have put MSG or Ajinomoto as something that's optional, and I regard dashi powder in the same light.
In Japan, more and more households are turning away from dashi stock powder for health reasons, especially in families with small children. I don't see any reason for people new to Japanese cooking to start out on the wrong leg by relying on an iffy convenience product.
[Edit:] I just realized I've addressed the same subject previously, but I think it bears repeating. Any book that proclaims that it's 'healthy' while using MSG or dashi powder for every recipe, obviously isn't up on the healthy-eating trends in Japan at least...