Gives a 25% bonus (10% in Audiosurf 2.)
About the only strategy for clean finish is to look ahead when you get near the end of the song, and not get blocks unless you can see that there is a match. The other thing you could do is play the song more than once and remember where the blocks are near the end.
With Pointman, you can grab hold of a paint bonus and release it before the song ends. If playing with Eraser, you can also "clean up" your blocks at the last part of the song, as long as they match. Vegas can save randomly generated paint powerups until the end.