Throw all those rules out the back door and write whatever you feel like writing.
I think all rules ever made by writers, even the evil point-of-view rules, the backstory rule, or number of characters rule can be broken. My God, has anybody read Beach Music by Pat Conroy? Talk about a rule breaker! His books are a category far and above those who write the rules. Beach Music is a masterpiece. Of course ... that's Pat Conroy--I know, I know--he's pretty much able to do what he wants. After all, he's no longer considered a "first-time" author.
Still, I've got a shelf longer than my left leg, full of books on writing ... and I've yet to find anyone with all the answers.
However, the tension-on-every-page rule is a pretty damn good one to follow if you want to attract today's "weekend only" reader. Or, "I only read on vacation" reader. Better yet, "I can't find time to read my Bible, why would I want to read a book?" reader. Then there's the vegging couch-potato that reads nothing but the Sunday comics, the TV Guide, and the menu at Dennys. Not much you can do with that.
And yet, tension is what makes a mediocre book a great novel. It's the reason for telling the story--tension and the effects it has on the reader and on yourself as an author. If YOU don't feel tension as you write your story, you're not going to sell many copies of your hard work. Nobody's going to care about your story, your characters, the reason you wrote it.
Aside from the rule of tension, I say write how and what you want, as long as it's a damn good story.
I think a writer's got to enjoy writing whatever he or she writes--not an intellectual satisfaction in their research that turned their prose into something credible. But a writer must find a way to string together words that turn into amazing plots, sympathetic characters, exotic places, and interesting dialogue.
You've got to write what blows your skirt up--and find a way to blow up your reader's skirt.
That's all. Enjoy the process ... God knows the crap that's published. Every once in a while, a flower pokes through a cow patty and get's noticed. I hope it's your story.
Blessings to you and yours.