In many simple elongational flows realized in existing rheometers under the assumption of constant extension rates, the sample radii or heights vary along the axis. This is mainly due to inertial, surface-tension and drag forces acting on a sample subjected to real experimental conditions. Then, the velocity fields are non-uniform in the sample cross-section and the presence of additional shear stresses is necessary. On the basis of the concept of flows with dominating extension (FDEs) we try to discuss the real shapes of samples, the relations between the sample shapes and the velocities, forces and elongational velocities, and the sensitivity of the flow considered to external disturbances. It is shown, among other things, that under realistic conditions the effect of inertial, surface-tension and drag forces is rather small, if their values are moderate.