Actually, you usually will have MORE TRACTION when decelerating slightly than when accelerating slightly in a front wheel drive car. Dufarge (above) is not correct. I'm not saying this just to trump Dufarge, but rather, for clarification and ACCURACY.
You can use your B gear any time that's favorable to regeneration, within Toyota's recommendations, and not merely in favorable conditions.
A front-wheel drive car loses traction slightly during continued acceleration (increase of speed) because the center of gravity is resistant change (thanks to inertia) and the chassis is tilted slightly rearward, placing more weight on the rear wheels, and thus, more traction there... and LESS WEIGHT on the front wheels, and, AS REQUIRED BY PHYSICS, less traction (suspension designs try to compensate for this, but can only do so partly, as the LAWS of PHYSICS cannot be bent or broken.
Now to the FACTS regarding deceleration (slowing down) by using the B gear to let your motor/generator absorb some of the energy, and convert it to electricity it sends to the battery: JUST DO IT.
Why? Because a deceleration rocks the chassis slightly FORWARD, placing more weight on the FRONT WHEELS, which not only do the driving -- they do the STEERING. And the more weight you have on those wheels, the more theoretical (and thus ACTUAL) GRIP you get, giving you more control.
The ABS system jumps in to help out here when ICE is present, to restore traction within hundredths of a second after it is lost.
Wanna test this out empirically? Take the same Prius and put it on the same slippery grade. (Closed course, professional driver, don't do at home, etc.) Measure traction loss going uphill and then downhill (the grade accurately simulates acceleration and deceleration). You'll have far more control in a FRONT-WHEEL-DRIVE during deceleration.
BOTTOM LINE: Use the B Gear whenever Toyota recommends, and don't be afraid of loss of traction, at least not due to a misunderstanding of fact about traction.
So, unless Toyota tells you NOT to use B Gear in slippery conditions, go ahead and use it. All other factors aside, you'll INCREASE traction to your steering wheels -- a GOOD THING.
(Just a guy who works as an engineer on similar issues of traction on designs for larger, but similarly principled, mechanisms and vehicles.)