It is standard practice to use a wood block as a guide when cutting with a reciprocating saw. But if the block is on just one side of the blade, either above or below, the blade can still wander beyond the cutline.
I began to search for a way to guide both sides of the blade recently when cutting the tops off some old studs prior to installing a header. As shown in the drawing, I simply took a scrap of 2x4 and made a square cut almost all the way through it. Then I clamped it to the old stud with the square cut aligned with the cutline mark on the stud. The cut guided the blade straight and square.
Don Mathis, Macomb, IL
From Fine Homebuilding 182, pp. 24
November 1, 2006