The short answer is yes (usually), but you don't have to, and it might not matter anyway.

It's not just Acorns; with online financial services for consumers proliferating, it's a question that will crop up more regularly.

Let me explain.

Acorns has a feature that lets you see your recent bank account activity, round up to the nearest dollar on individual transactions and add the virtual spare change to your Acorns investment portfolio.

It's a great way to save little and often, but in order to use the feature you need to enter your online banking login details on the Acorns site.

You can still use Acorns without it. The round-up function won't work, but you'll still be able to transfer money via direct debit.

But with many fin-tech services, it's not optional. And if you do provide your login details to Acorns or anyone else, you're stepping into a legal grey area.

First check with your own financial institution, because approaches vary.

For example, CommBank and NAB say customers are responsible for any loss if they give out their login details, while Westpac gives customers more wiggle room. Westpac does not explicitly state it's a breach of the terms and conditions, and advises customers to take "reasonable steps to protect and not share their access codes with third parties they do not know or trust".

This is not the end of the story because regulation – specifically the Australian Securities and Investments Commission's ePayments code – over-rides the banks' terms and conditions.

The code states that online banking users can give their credentials to a third-party aggregator service explicitly or implicitly endorsed by the banks (emphasis mine).

Acorns says this means customers are in the clear because Acorns uses a US software service called Yodlee that is also used by the banks. You're not giving your login to Acorns, you're giving it to Yodlee, and then only in encrypted form.

The questions then are "do you trust Yodlee?" and "does your bank endorse Yodlee?", or perhaps "can you argue that it implicitly endorses Yodlee?"

Yodlee has been operating for 16 years, and is considered to have high security standards. It is set up to scrape information from your account but shouldn't be capable of withdrawing money.

It's used not just by Acorns but broadly across the financial services industry, including by ANZ Bank, robo-adviser Decimal, peer-to-peer lenders such as RateSetter and SocietyOne, non-bank lenders such as Liberty Financial and Nimble, and online accounting software companies such as Xero (which is replacing Yodlee with authorised bank feeds where possible).

If your bank uses Yodlee, you've got a pretty good argument that your bank implicitly endorses the service.

CommBank, Westpac, NAB say they don't endorse or use Yodlee, but ANZ uses Yodlee for MoneyManager, a service that lets customers see all their bank accounts (not just ANZ accounts) in one place. This means ANZ is effectively encouraging consumers to share their login details for other banks with Yodlee.

An ANZ spokesman says the bank only authorises customers to give account access to third parties in cases where the bank itself "provided or referred the account services to the customer". The bank didn't have a ready answer to whether that meant Yodlee more broadly was in or out.

A Yodlee spokesman says consumers should have control of their own financial data, and strict terms and conditions were "causing uncertainty and confusion and potentially serving to stifle innovation and progress".

After spending so many years schooling users not to give out their login details, it's hard for the banks to suddenly turn around and be cool with it.

On the other hand, banks are keen to wear the mantle of innovation and look like they're supporting rather than crushing the fledgling fin-tech industry.

One banking insider told me the only real power that consumers have to push their banks to be allowed to use fin-tech services is to vote with their feet and do it.

Of course, that's risky and only you can decide.

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