Generally we play by the rules of grammar in creative writing because readers will be put-off or confused by our failure to abide. Grammar is the fabric of the language. There is no English without English grammar. Even a nonsense poem like Lews Carroll’s Jabberwocky is grammatical, even though it deploys many invented words.
But some grammar rules are more rigid than others. In “John hit him,” “him” must be someone other than John. “The house blue” is not grammatical because English sentences cannot omit verbs of being. We might try to parse it as a phrase, “The blue house,” but word order prohibits that reading, just as it prohibits reading “The book read Jill” as Jill reading a book.
Some sentences, so-called garden path sentences, are grammatical only when parsed properly: “The old man the boat” seems like nonsense until we construe “The old” as the subject and “man” as the verb. Then it makes perfect sense.
Comparatives (e.g. happier, bluer, better, more thankful) and superlatives (e.g. happiest, bluest, best, most thankful) are an interesting class because we form them in two ways, comparatives with -er/more and superlatives with -est/most. There are rules for when to use each construction and breaking those rules is improper grammar, but flouting them does not confuse readers. “Mysteriousest” is ridiculous, but we know what it means.
Typically ungrammatical usage will make readers pause; bad grammar works much like paradox. Consider when Lewis Carroll puts “Curiouser and curiouser!” in the mouth of Alice. We’d perceive the humor in this interjection even if he hadn’t added the aside, “she was so much surprised, that for the moment she quite forgot how to speak good English.” (It seems that Lady Grammar is such a harsh taskmistress that even a writer as wily as Carroll felt the need to explain himself!).
Flexibilities run deeper. “Cleverer” is bad grammar, but it’s, well, cleverer than “more clever,” which is grammatical. Here proper grammar is seems uptight.
Consider this sentence from one my stories: “He knew all there is to know from the most clandestine kiss between lovers in the heaven of fragrances to the mewling of the wretchedest demon in the bottommost hell.” “Wretchedest” is bad grammar (although not so bad as “mysteriousest”), but the sentence already has two instance of “most” (including “bottommost”), so “most wretched” is too much.
To this poet’s ear, “most wretched” feels like a one-two punch to the reader–two emphasized syllables in a row. The cadence of the sentence is lost. “Most wretched” would be like H.P. Lovecraft writing the first line of his oft-quoted couplet as “That is not dead which can eternally lie.” “Eternally” is better grammar, but the sentence stumbles.
In Sanskrit poetry, grammar is frequently disregarded or words are added for no other reason than to preserve poetic meter. According to lore, Sanskrit poets are tortured in hell by the unmetrical syllables they wrote in life. A strange notion, but I get where they’re coming from.
Compare “greener, more real, more alive” to “greener, realer, more alive” to “more green, more real, more alive.” The former is widely considered proper grammar because “realer” is uncommon usage (even though “real” is just one syllable). But the latter two formulations are more felicitous. The middle case, has forward motion; emphasis lands at the end. The last is stately; emphasis lands on the successive “more.” For all its grammar “more real” in the first case strikes this writer as another one-two punch. It reads like pandering to Lady Grammar.