Don't use history as your guide on how the next round of quantitative easing from the Federal Reserve will affect your Treasury portfolio.

As expectations grew before the first round of quantitative easing in 2008, known as QE1, and then the second in the fall of 2010, demand for Treasuries rose and their yields fell—as you'd expect from programs that included direct purchase of U.S. government securities. But once the purchases began, prices fell (and yields rose) as investors steered the Fed's newly unleashed liquidity into riskier assets and sold down their holdings in safe-haven Treasuries.

The 10-year yield rose 1.46 percentage points between the start and end of QE1. It rose 0.38 through QE2.

But now that Fed Chairman Ben Bernanke's remarks and a weak August jobs report have raised the likelihood of QE3—a move that could come as early as the central bank's Sept. 12-13 policy meeting—investors aren't so sure that a subsequent selloff will be forthcoming.

That's partly because doubts about the effectiveness of more monetary stimulus are seen limiting the kind of cheer that spread through riskier markets in the earlier rounds. This, in turn, could limit the damage done to Treasuries and bolster their role as a sustained haven in times of economic stress.

Rates strategists at Bank of America Merrill Lynch wrote recently they suspect QE3 will not be as bearish for bonds as the first two rounds because of the "perceived ineffectiveness of monetary policy."

Many investors, though hopeful of more Fed stimulus, acknowledge that the U.S.'s economic bottleneck doesn't lie in borrowing costs—which are now near the lowest on record for various markets—but in getting individuals and businesses to take advantage of the low rates. And that's an issue beyond the Fed's reach.

Inflation expectations also remain contained, which helps to keep Treasuries in favor. Rising prices are a threat to bondholders because they eat into the value of fixed-income payments.

"The question isn't will the Fed act, but more will these actions have any meaningful impact on markets and then the economy," said Chris Molumphy, chief investment officer for Franklin Templeton's fixed-income group. "As we move through each round, the potential impact absolutely diminishes."

Without a doubt, yields are lower since the Fed started putting its hand in the market. And rates won't likely move much higher as long as the Fed remains involved.

But there's only so much credit the Fed can claim for keeping rates low when this unconventional program's record in boosting investment in the real economy and producing jobs remains questionable.

Should the cheer over QE3 turn out to be muted, it could very well double up the demand for Treasuries. Bond managers expect new waves of fear-driven buying in Treasuries as China's economic slowdown and Europe's debt crisis draw greater concern. Most think the European Central Bank's new bond-purchase plan will have only a temporary calming effect.

"I like Treasuries as long as the Fed announces QE3, as I don't believe yet that the ECB can contain this crisis," said Matthew Tuttle, chief investment officer of Tuttle Wealth Management. Though on one hand "flight to safety is ebbing as people get more confidence that the ECB has their act together," he says, "On the other hand, you could see the Fed start up the printing presses and buy bonds."

APPETITE FOR TREASURIES FADED Thursday after ECB President Mario Draghi delivered details of a long-awaited bond-purchase program. But economic worries on the home front resurfaced Friday, with a weak August employment report working up demand for safer assets. It all left the market starting September on a down note. The 10-year Treasury yield finished at 1.66%, up from 1.56%.

CYNTHIA LIN writes about the U.S. government bond market for Dow Jones Newswires.