2006, 156K miles, recently started bucking when ICE engages

When were the iridium spark plugs last changed?

 

If the plugs are marginal that might result in the engine taking longer to start than it should, causing the symptoms you described.

Another possibility is that the fuel line pressure might be slightly low, again resulting in the engine having trouble starting immediately and smoothly. The spec is 44-50 psi when the engine is at idle speed. Five minutes after the Prius is made IG-OFF, the fuel line pressure should be at least 21 psi.

 

I have this issue aswell, however my car is a 2009 with only 61k miles on it (Just bought it used 5 days ago). From what I have read so far it is normal for the engine to be a little rough because of the way it is tied into the drive axle. However, I am most certain the roughness I am feeling is beyond normal. I too experience it mostly when sitting idle in the car. I drive for uber, so I spend a lot of time sitting in my car waiting for rides (as I am now). Should a 2009 with 60k need spark plugs already? I thought that was typically a 100k thing, and maybe even more for a prius as the ICE isnt always running.

 

120K miles is the replacement interval for Gen2. CARB states list a 150K mile replacement interval. This is what mine look like at 120,631 miles.
Spark Plug pictures after 120,631 miles | PriusChat

 

to evaluate the "usual" hybrid "issues", i would, for peace of mind, test out other 2nd gen prius perhaps at other dealers or carmax etc. as long as they are relatively well maintained you can see what your new car should be acting like. usually the prius is very "smooth" in transitioning. the only rough transitioning hybrid of note is the Nissan Altima hybrid but even then there isn't a hard jerk just a louder ICE noise and engagement. i would definitely get your transaxle fluid changed though. 60k is very optimal change interval.

 

The normal spark plug replacement interval is 120K miles. However if the engine is running rough, then it is certainly easy enough to remove the iridium spark plugs for inspection.

Maybe some water got into a spark plug well, causing misfiring. Inspect the plugs and the spark igniters and replace as needed.