Usually you can get a decent trace that you can clean up. However, sometimes an object with a lot of light areas just won't trace well enough to get a usable cut line or it has so many nodes and is distorted enough that it would take hours to clean up the nodes. I will admit that when that happens I normally just take the raster file into Photoshop and work on it there and make a blackout version to trace. I do know that everyone doesn't have Photoshop so there are alternatives, especially when your object is not too complex. Actually you could use the drawing tools in Silhouette Studio to draw around anything, but adjusting the nodes on a complex object would take a whole lot of time.
I got a file from someone on the Yahoo Silhouette Users Group to see what I could do about getting a good trace. Well, I couldn't get one I thought was usable. It was a very simple gable box pattern so I just drew around it and used the node editing tool to clean it up. It seemed a good thing to share how easy this is to do on a simple object so, of course, I felt compelled to do a video...and here it is. Remember if you watch it on YouTube you get a bigger screen.