Blazars are characterized by the existence of brightness variability in all observed wavelengths occurring in very short time scales. The origin of the extreme activity is probably related to a relativistic jet that lies very close to the line of sight. Since the radio to X-rays emission in blazars can be associated with synchrotron radiation, multiband polarimetric data are an important tool to investigate the nature of the components associated with the brightness variability. In this work, we present a study of the optical polarization variability in the TeV BL Lacs PKS 2005-489 and PKS 2155-304. The data were taken between 2003 and 2006 with time resolution from a few hours to a few years, complemented with data obtained in the literature, when available. For both sources we analyzed the incidence of variability at different time scales; the existence of correlations between total and polarized flux and the existence of preferential values of polarization. The combination of results favours a scenario in which the origin of the variability is associated with shock evolution along a quiescent jet, which changes its of orientation in time scales of decades.