To be honest, if it was two years ago, I'd have said the 10-20 range myself as well... However, latestly I've been on about 24mm's at the widest, and I'm far happier with the slightly cropped results. Very few wide angle shots actually come out making a big impression on me anymore... Just too much distortion. For you, I'd said, the 18-50 is a great lens, that said, an UWA is sometimes exactly what is needed.
On my 5D, I foundmyself using my 24-70 the vast majority of the time, second being my 16-35 (equal to 10-23ish) and my 70-200 about the same number. Remember you can always panorama it too. ^_~
Here are some photo's from my trip in August last year, use that to go from.
Just to add my bit as well for photography equipment in Jap-land.
Yes it IS cheaper provided you know where to go.
The point to remember is BIC Camera and Yodobashi are massive retailers, and sell at retail price. No cheaper. You do however, get 'Points' for shopping at either of these two. These points can be used as a discount. Basically for every $10 (or 1000yen) you spend you get a $1 (or 100yen) of points that can be used for further discount... Actually, I think it's closer to 5%... Can't remember, I haven't shopped at either for quite some time. The bonus is that the lenses they sell have international warrenties guarenteed.
If however, you don't care about the points, much cheaper prices can be had at different places.
The place I prefer and recommend to EVERYBODY who is going to Japan is Fujiya Camera in NAKANO (Not SHIN-Nakano, or Nagano). Nakano, is reachable by taking the Chuo or Chuo Express from Shinjuku (one stop on the express). When you leave I it's the North exit in the terminal (there are only two exits, and the one you need is the one that dog legs) then turn right. You should see an entrance to a mall, across some form of promenade, with a taxi route around it. Go into the mall, and as you do, keep an eye for the alley on the right hand side that goes outside. There's a fish store on the left, and a guy selling Octopus balls on the right. Head down the alley and you'll see a store (last time it was yellow) with a whole bunch of camera flyers in the window. Go into it, and you're there.
EDIT: As of 2011 and still into 2012, they're doing construction work on the promenade and though it still has a path through it, it's a little uglier.
The sell SLR and compact stuff on the lower level, with medium format, and large format upstairs.
Even for memory cards, they are as much as 50% the price of Bic Camera. They're also not a dodgy retailer, they're a professional retailing outlet specialising in second hand equipment, but they also have new equipment in the central cases. They practically go through the majority of the EOS line up in a week in their second hand cabinet. The prices are also signficantly cheaper even though you can't get duty free there compared to the likes of BIC Camera and Yodobashi.