I could argue you always need a proofreader and you should in fact hire me to check your every written word. That would be preposterous and unsustainable as a business model, even if people were convinced!
We all post a lot of text these days. There's WhatsApp (other messaging services are available), Facebook, Instagram, Snapchat, Twitter, of course LinkedIn and loads of other platforms I don't have. We send emails, we text the other half about what's for tea and for business we constantly communicate in the form of words.
So what should you proofread? You can't get every tweet and Facebook post checked, you'd never post anything! Unless you're a proofreader or writer it's unlikely that grammar or spelling mistakes in everyday posting will affect your brand or business. Arguably even longer posts and blogs needn't be perfect (did I just say that? I feel a bit sick now..) IF they are well written. If you have a great style and lots of interesting things to say people will forgive a few grammatical errors. Although don't think they won't be noticed, some people will always notice.
That's a big if though and I actually have a lot of respect for some great bloggers and social media influencers who have terrible grammar but don't care and it's almost part of their brand. Even if it does make me want to get my red pen out! But if you're offering any type of writing service, including web design, social media management or marketing it might be harder to find clients that don't mind about those mistakes.
Websites are the biggest place I think you really need to get it right. That text stays there and it doesn't scroll away down a Twitter feed, people read it carefully if they want to know about your services and that website is as close to you as some potential clients will ever get. Obviously my eagle eye spots even tiny errors but you'd be surprised how many websites are littered with quite obvious mistakes.
Anything printed you're in even bigger trouble, at least a website can be updated relatively easily (in theory), print is permanent. I'm sure we've all printed something then realised there's a mistake. I made my wedding invitations for my first wedding and nearly invited everyone to a gold club instead of a golf club ..... It's so hard to see the obvious when it's your own text, the brain sees what it wants to. Nothing worse than reading a self published book when they've skimped on the proofreading.... Because trust me you can tell (and yes of course there are worse things, don't get me started on menus!)
So do you need a proofreader all the time? No of course not and you may choose to never use one, but please get someone with some skill in the area to read through your blogs and web copy and anything you are getting printed, it may cost you not to. Grammarly (free online grammar checker) and the spellcheck in Word are very useful tools but they won't spot everything.
And if you need a proofreader anytime soon you know who to call, *points to herself* 
