Very.

I assume you know that InDesign lets you automatically align text to the baseline grid?

Have a look at this Typophile thread.

At my point of view, it's important to use baseline grid for at least to avoid to see the text not align on a duplex document. Also I find it's easier to read if the text is align from one column to an other one. Your eyes don't jump from one line to an other one.

As long as the main text matter is on the grid, one can always vary the position of subs, captions, headlines etc in respect to the grid. Consistence is important in this (Eg paragraph heading always 3 pts offset, etc.).

Think of it as a musical rythm. Elements should enter and even sometimes exit in beat with your established pulse (the body copy), and a syncopated break should be consistent through-out. After a break you return to the pulse again.

Hi Jonathan... yes I am familiar with the "align-to-grid" feature.

Sometimes I try to get everything aligning to the grid, but end up liking my page layout better when some elements do not align (ie Headlines, Sub heads, etc).

I like the musical rhythm metaphor!

I used to think I had to get the software to automate vertical alignment for me in any circumstance, but in my experience of producing traditional books the effort hasn't been worth it. When first using ID I wrestled with baseline grids and I would get caught out by unexpected effects, often to a frustrating degree; it seemed, to me at least, more trouble than it was worth, or perhaps I was missing something fundamental. Anyway life was too short: Now I simply decide carefully on a body text line pitch, and ensure that any other settings, whether display, quotes, tables, pictures, captions, whatever, always use a multiple of that pitch in their overall height (not their line pitch necessarily), which is not hard to achieve using a combination of type size, leading, space above/below, baseline shift. It's fairly easy to make the body text pick up where it left off, and so align in spreads and back up properly, without making everything else look as if you just hit the return key between lines. For traditional books this is easy enough to set up and adhere to. And when the content is finalised I will often just 'nudge' the text into alignment if I have to, eg under a rogue graphic, where this does not visibly alter the overall consistency of the vertical spacing. When it's printed, who cares how it was achieved? My objective is a printed book, not mastery of the technology if that proves distracting.