My 17-40 has the hood on permanently. and any pics I take of people indoors with it on, I also use my speedlite. the on-board flash gets obscured by hoods even on smaller lenses.
My thrifty 50 I don't have, or need, a hood for. Certainly I wouldn't spend money on one, especially not the Canon one....
The 70-200 is a great portrait lens but not all that useful indoors, unless you have a bit of room, and I probably wouldn't use the hood indoors because it just makes the whole thing too long.
I haven't gotten my hands on the sigma yet - it's 'in the way' as my friend Tareq so eloquently puts it _ but I would probably not use the hood indoors with that lens either.
So I guess the only reason to use a hood indoors is because you can't be bothered to take it off - at least in my case. 