That is, of course, just my two pennies worth...I'm sure others may disagree!
Well,
of course I'm going to disagree with
you, Hoopy; it's
absolutely required!
Seriously: It depends on the type of story you're writing. Flashback that is simply "infodump" can slow a story down terribly; on the other hand, I've read plenty of stories where the "present" was only an opening paragraph or page or two, and a closing of about the same amount -- the entire rest of the story was "flashback" and made the story work, so that the ending had genuine punch; had it been told in chronological order, it would have completely lacked that emotional impact because one would have been prepared gradually for it, and would have seen what was coming.
One thing to keep in mind is that you should have firmly in your own mind the interconnection of the two, how they intertwine, so that you can keep the narrative tension at a high level. One way to do this is indeed to go back and forth between the two, while winding the connections tighter the closer the two stories come to each other in respect to time -- in other words, as the "older" story approaches the time of the "present" story, the tension between the two should steadily increase, to the point where, when they meet up in point of time, they reach a crescendo that forms at least a secondary peak for the reader (if you are carrying the tale past that point, in which case the actual climax of the tale should provide the greatest emotional peak).
The thing to avoid is, as noted "infodump" (just having the "flashback" to provide historical background, dry personal information, etc.); rather, see it (and present it) as integral to the story happening now -- you don't necessarily need to refer to the "present" story, but have enough small hints that the reader can see where one leads to the other ... it's that confluence of stories that can be used to heighten your narrative tension and "turn the screw" ... get your reader anticipating, wondering how these are going to resolve themselves into the present situation and beyond, etc. In musical terms, it's like a fugue, where you introduce one theme at the beginning and then, while that one continues on, yet another theme is introduced in another voice, and as the main orchestra picks up each theme another can be introduced, until they begin to interweave and feed each other to a complex, emotionally satisfying blending of the whole in a final restatement of the original theme, but informed by all the others.
Follow what your story demands to keep that emotional tension, and find a way to insert the information in such a way that it aids in that effect. In the end, with such things as this, that's the only rule that really counts.
