Congratulations!
All lenses have a sweet spot, but the 17-55 f/2.8 is very sharp even wide open. When I got mine I shot an ISO 12233 resolution chart at various apertures and found it was sharpest at f/5.6, but f/4 was really just as sharp. Tiny difference. And f/2.8 was again a tiny bit softer.
The only real difference is at f/2.8, where the upper left corner is soft because of curvature of field. Not the other three corners, though. This was on my particular copy of the lens and there will certainly be variations between copies.
Basically, I set aperture for light conditions and desired DOF, not worrying about sharpness. If it's at f/2.8, it's still very very sharp.
-js