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.
, Macomb, IL