We derive an analytical expression for the variance of the timing jitter of a soliton transmission system using sliding-frequency guiding filters, taking into account the third-order filter term. We show that the variance of timing jitter is significantly increased by the sliding action As a consequence of the third-order filter contribution, the timing jitter is lower in a system with down-sliding than it is in a system with up-sliding at the same sliding rate. Consequently, in what concerns the timing jitter, it is advantageous to use down-sliding.