What exactly is grammar?
According to Merriam-Webster:
"Rules of a language governing its phonology, morphology, syntax, and semantics; also, a written summary of such rules. The first Europeans to write grammar texts were the Greeks, notably the Alexandrians of the lst century BC. The Romans applied the Greek grammatical system to Latin. The works of the Latin grammarians Donatus (4th century AD) and Priscian (6th century) were widely used to teach grammar in medieval Europe. By 1700, grammars of 61 vernacular languages had been printed. These were mainly used for teaching and were intended to reform or standardize language. In the 19th–20th centuries linguists began studying languages to trace their evolution rather than to prescribe correct usage. Descriptive linguists (see Ferdinand de Saussure) studied spoken language by collecting and analyzing sample sentences. Transformational grammarians (see Noam Chomsky) examined the underlying structure of language (see generative grammar). The older approach to grammar as a body of rules needed to speak and write correctly is still the basis of primary and secondary language education."
So, I guess that includes punctuation and word choice. Right? ;-)
When should you NOT worry about grammar rules?
During the writing of your first draft.
If you obsess over little details during the first draft phase, you may never finish it. During revisions you can clean up grammar mistakes, make better word choices, and polish your punctuation.
And because it's important to know, here's a list of words that rhyme with grammar (also from the Merriam-Webster link above):
clamber, crammer, glamour, hammer, jammer, lamber, rammer, scammer, shammer, slammer, spammer, stammer, yammer
Please, while I know you're not a spammer and because you don't stammer, go yammer about grammar with your hammer and your glamour.
What's your favorite (or least favorite) grammar rule? :)