In addition to the "flip and fire" approach, there are a number of other factors that contribute to sharp, crisp lines in a piece made with strips of glass laid on edge and fused.
Smooth glass will fuse straighter than strips of textured glass. The individual strips fit closer together, leaving less room for lines to wander and create a wavy appearance.
The quality of the cut of the strip is important. Straight strips with right angle edges and no flares make for crisper lines.
The thinner the strips, the less opportunity for movement in the fusing when they are placed on edge. Ideally, the strips should be 6mm wide. This is the thickness that glass tends to take up when full fused. The greater the width beyond 6mm, the less likely the lines will be straight.
The viscosity of the glass affects the crispness of the lines. A glass that is less viscous will tend to be more wavy than a more viscous glass. E.g., black glass, a less viscous glass than white, will tend toward waviness more than the white. This is not a variation between manufacturers; it is a variation within a compatible range of glasses.
The firing surface will have an effect. Firing directly on a kiln washed shelf will give crisper lines than firing on fibre paper of whatever thickness.
Damming the composition before firing will produce straighter lines. The dam holds the strips in place during the heat up and restricts any flow that would be caused by strips thicker than 6mm.