Opt-in is always a good idea, regardless of the legalities, for one simple reason: people who actively opt in are more likely to open, read, and act on the emails you send to them.
People who are 'tricked' into receiving email from you through subclauses in the terms and conditions or pre-ticked boxes (i.e. the 'opt-out' system) aren't as likely to buy from you, regardless of your good intentions. Here's what MailChimp says about opt-out email:
"It's sort of a scummy way of doing it, but technically it's legal. We highly recommend against it, because you'll end up with tons of people who don't understand how they got on your list, who won't read your emails, and who will send complaints to the anti-spam authorities to get your server blacklisted. It's yucky, so stay away from it."
So yes, you should specifically ask permission with a separate tick box that's unchecked by default, and not assume that people who've agreed to your terms and conditions also agree to hearing from you. They probably won't read the T&Cs anyway. This is the case even if you don't ever expect to send them anything, but I would suggest that you send some news at least once a quarter so that they don't forget who you are when you do have something absolutely essential to say.
In case it helps, this is how I collect email addresses on all of my forms:
About 90% of people who contact me tick the box to receive the newsletter. I like to think it's because it's non-threatening, playful, clearly states the volume of mail they're likely to receive, and because it reminds them I won't spam them, but they might just be curious about what 'jolly cool stuff' I send.