Make a list of all your money worries. Where does “Too much spare cash” rank?

If having surplus cash piling up uninvited doesn’t seem like a pressing concern, then the entrepreneurs running today’s leading automated investing services would like a word with you.

Companies such as the pugnacious rivals Betterment and Wealthfront would apparently rather snipe at one another than express agreement. Yet these fast-growing services – which offer easy, low-cost online investment accounts – strike the same note when it comes to the “risks” of having too much cash uninvested in the markets.

Betterment, in fact, is now launching a new feature that will constantly monitor a customer’s bank account and automatically move any cash above a preset target balance into his or her investment portfolio.

Called SmartDeposit, the tool fits with this burgeoning industry’s focus on automating each individual’s retirement-savings plan and monitoring it for adherence to investment models steeped in long-term market simulations and behavioral-finance theory.

Wealthfront CEO Adam Nash, meantime, assailed a new no-fee automated-investing service from Charles Schwab Co. (SCHW) in March for its allocation of at least 6% of each portfolio to cash.

While Schwab contends a cash reserve is prudent risk management, Nash focused on the potential lost market appreciation over an investing lifetime due to this “cash drag” – and the fact that the cash would be placed in bank deposits on which Schwab earns interest.

One interesting element of Betterment’s SmartDeposit idea is that it runs somewhat counter to a dominant trend in the brokerage industry in recent years of “sweeping” excess cash from brokerage accounts into FDIC-insured deposits. While this can be of slight help to clients in times when deposits pay interest, it’s of substantial benefit to the firms themselves, which can earn a spread on those customer cash balances.

Overriding human emotion

The hostility of the so-called robo-advisors toward cash gets to the core of their approach to money management.

These companies distill market history and finance theory into multi-decade return assumptions for all asset classes, which then feed algorithms to drive individual asset allocations -- integrating clients’ risk tolerance as determined by brief questionnaires completed in the account-opening process.

Their models reflect the reality that long-term investing success is determined mostly by maximizing saving over long periods and investing the money at low cost.

Betterment and Wealthfront have grown quickly from small bases, as their message captures both the powerful trend toward low-cost index-fund investing and the broader encroachment of software replacing or mediating among human-provided services.