Ducan's advice is true for all wide angle lens. Trapezoid distortion (perspective/tilt) is easy to correct for in Photoshop etc. Pincushion and Barrel Distortion is not so easy especial when mixed with trapezoid.
It appears the the 17-85 is know for it's heavy Barrel distortion at the wide end.
http://www.photozone.de/canon-eos/17...review?start=1
The 10-20 does exhibit some barrel at the 10mm end but looks pretty good from 14mm upwards.
http://www.photozone.de/canon-eos/30...review?start=1
I have the 10-20 and I can't say that barrel distortion has been a problem when photographing buildings compared to using the 18-55mm kit lens at the wide end. The kit lens seemed to introduce a S-shaped bow which meant you could not get all the horizontal and verticals straight.
The only way to sort these Barrel and Pincushion problems in software is to obtain or create a profile of errors for your model of lens. Then it becomes a simple matter to correct the tilt.