I find it easier to use the regular vector rectangle tool (U) than the special rounded rectangles, for exactly this reason. With a regular rectangle, you can use two methods to round it:
- Set the "Roundness" in pixels or percentage in the Properties panel.
- Or ungroup the rectangle into a path (Ctrl+Shift+G) and the use the Fillet Points command under the Path panel.
The Fillet method is probably more convenient if you're already working with paths (maybe you're making a complex path where you need to punch a rounded rectangle out of another shape), otherwise the first method is probably a bit faster. You can reverse the effects of Fillet with Sharpen Points, which is the button right next to it in the panel.