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As a Prius owner for the past 3 or so years, I can confirm "terrible" mileage with the first 5 minutes of driving, not 10 minutes though. I say "terrible" because its still above what average people get on a normal basis.
I have a commute that is just under 15 minutes (7 miles). My first bar on the Prius MFD (they're broken down in 5 minute segments) is normally around 25-30 mpg. By the time this bar is up, I've gotten up to temperature and can start EOCing if my speed is still below 40 mph. My second bar (minute 5 to 10) is normally around 75 mpg in summer (I travel a 45 mph road to work and its slightly downhill). My last bar, if it pops up, is normally pegged at 100+ mpg as I'm back into the neighboring town and doing under 40 mph where I can P&G.
So, for a 20 minute commute you won't have any issues.
As Dain said, ALL vehicles (besides PH/EVs) get horrible mileage in the first few minutes. This isn't really a hybrid specific thing. If normal cars had the same MFD as the Prius I'd bet they'd see a very similar thing.
A block heater can certainly help this, but I still rarely see 35+ mpg in the first 5 minutes unless my engine is already pretty warm. The one situation where I have seen high mpg in the first 5 minutes consistently is if I get in the car and almost immediately get up to a decent speed (45+ mph).
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