(Well, all right. It’s more like a guideline….)
Don’t violate natural chronology in your story without a really good reason.
I’m not going to say you should never do it, because sometimes there’s just no other way to get necessary information across to the reader, and because sometimes the violation or alteration of natural chronology is the main effect or whole point of the tale. But before you send your plot into a temporal zigzag, think long and hard about whether or not those conditions apply, because the failure modes can get ugly.
Possible failure mode number one: Your reader may get lost and confused by all the flashing back and forth, and give up on finishing the story. If you’re going to use the flashback technique to deal with important events in the story’s past that deserve a full and direct presentation, be extra-careful to provide your reader with markers and signposts. Give them details (verb tense changes, time and place references in the narrative, even explicit chapter or section headers if you think you need them) to let them know they’re heading into the past, and more details on the other side to let them know they’re coming back out of it. No, it’s not mollycoddling your readers to do this; and you aren’t building a mental obstacle course for them so they can get a gold medal for running it, either.
Possible failure mode number two: your readers may get more interested in the past of your story than in its present. This happens a lot with the sort of books where the main plot arc involves finding out the deeply buried family secret, or the suffering hero’s secret trauma, or the dreadful thing that the students of Professor Thingummy’s Early Western Drama class did during the summer of 1995. If you’ve got something portentous and dramatic like that lurking in the backstory for the hero or heroine to find out, you need to make sure you’ve got something even more portentous and dramatic going on in the front story, just to make certain that your reader cares enough about the story’s present to want to keep on reading about it. Otherwise, your readers are likely to read the interesting backstory bits and skip the boring front story bits, which will leave them with only half the book that you meant them to read.
(Yes, I admit it. I once co-wrote a book where a good portion of the plot involved a bunch of interpolated backstory bits. In my defense, I set them off from the main text in separate chapters and labelled them and timestamped them clearly, and ran them in their own proper chronological order. I thought it was necessary. And I hope it worked.)