In this post, Pete Brown talks about layout rounding, which has nothing to do with using rounded corners.
WPF now has a great UI feature that Silverlight got back in the SL3 timeframe: Layout Rounding. Layout rounding lets you specify that you want the shape to snap to the closest whole pixel. Rather than have a double-width gray line, you'll get a single-width black line, but it may be a partial pixel off from where you thought it would be. That's the trade-off, and it's one that many folks are willing to make.