i am working on a tumbling block right now too- pic's tomorrow :)
but anyway- i do alot of prep work- i cut my diamonds- then i sit with a ruler and pencil and draw the 1/4" line all the way around each one. it goes faster than you might think-and saves lots of time later. when i sew i stop right on the x's where the seams meet. no puckers, everything laying nice and crisp :)i will snap a couple pics of start to finish tomorrow to.
if you are using english paper piecing you need to stitch right to the points.