No one likes dealing with the Grammar-Punctuation-Spelling Stasi (GPSS)* but if your job is based partly or completely on communications it makes sense that you would attempt to get this right. Not everyone notices mistakes but those who do find it quite irritating and it can distract from the message you are trying to put across. Of course there are many grey areas, particularly in grammar, and the 'rules' are constantly changing but the fewer (non-deliberate) errors you make, the better chance there is of you being understood by more people.
Like music and art, an informed, willful breaking of the rules works better than just making mistakes, putting the occasional idiot savant and lucky error to one side. For one thing, the rule-breaker can judge how far outside the norm they want to take their audience. To do this you need to have a grasp of what is considered standard usage by the people you are writing to. And you need to decide when and why you want to break those rules.
There are pages and pages of help in this area all over the internet but I bookmarked this simple list on punctuation to help me in my writing. I'm not saying it's perfect, or even correct in every point (I am well aware of my own limitations in this area) but in it's favour, it's short. It's also American so as they say, your mileage my vary when outside the US.
For a bit of light relief, and opinions not necessarily held by anyone at MCA, David Mitchell discusses another element of the topic in this video.
* I have the Stockholm Syndrome when it comes to the GPSS. Having spent time in the virtual Hohenschönhausen for crimes against punctuation and grammar I have also been known to act as an agent of the GPSS.