If you look at some of very successful programmers and designers, they talk a lot about their work. For example, it's arguable that Magic's Head Designer got the job partly because of his strong commitment to community interaction, both because it made him a face, and because it improved his ability to satisfy the ridiculous demands of his entitled userbase. It's certainly arguable that Coding Horror and Joel on Software were what helped build StackExchange.
Obviously other successful programmers don't talk at all, at least not on public forums, but it's obvious that it can help your career far more than it hurts it.
So you have to ask yourself, do you fit that role well enough to benefit from filling it?
Are you interested in discussing your work with other people?
This sort of breaks down into "Are you interested in your work" and "Are you interested in writing about stuff online?"
If the former isn't true, this can be an opportunity to change that. For example, you can say "look, what I'm doing at the moment won't make for good branding because it's kind of dull and routine; I'd really like to work on Project-X, because it's the kind of thing I can really write some interesting technical/user-facing stories about it".
If the latter isn't, then don't. It's not your job, and if this doesn't seem like an opportunity, why do it?
What can you actually write about?
If they have very scripted things they want you to write about then they're effectively asking for control of your social media. If they want a bunch of dummy accounts, they can pay for them.
If they have a review policy before you publish anything, then that's great, but be aware that you may have to fight for what you write. But this way you'll be safe if anything you publish causes brand issues, because it got brand signoff.
If they don't have a review policy, then you need to either need to persuade them that they need one, or have them contractually stipulate that they can never fire you or take any action against anything you might publish online ever. Otherwise they're asking you to write stuff, but if you write the wrong stuff, you'll face disciplinary procedures. What's the wrong stuff? Well, it's anything they dislike.
Rule 1: Never risk anything you value on your ability to psychically intuit a complete stranger's whims.
What is your brand?
What face do you want to represent when people talk about you? If you're highly technical, your writing should be about the technical problems you solve. If you're really interested in your problem domain, it should be how the software solves real problems that are faced. If you really enjoy the environment you work in, it should be about the way you're managed and how it helps you solve issues. All of these have value to your company either via recruitment or sales.
Writing is a great way of being seen, and being seen is a great way of being valued. But you have to do it with your eyes open. It's great to tweet about the cool stuff your company's doing; some of the best Twitter accounts do just that. But it's only sensible to ask what you get out of this; and what you get out will very much depend on how much you're prepared to put in.