I know for a fact clonazepam decreases the utilization of 5-HT... but I do not know of other benzos affecting serotonin levels in any ways. Clonazepam is supposedly the only benzo that works well for HPPD and it is used because of its unique effect on serotonin utilization by the neurons. It is also used for depersonalization disorder and is first line because of its unique serotonin action.
Source: Chronic administration of the benzodiazepine, clonazepam, increased the number of [3H]-hydroxytryptamine (5-HT1) binding sites in the frontal cortex of the rat. The increase reflected a change in the maximum density of binding sites (Bmax) with no change in ligand affinity (Kd). Increased binding occurred after continued exposure (10 days) to large (5.0 mg/kg) doses of clonazepam. The changes in [3H]5-HT binding were regional in that they occurred in membranes from the frontal cortex but not the brainstem. The effects were also at least partially selective for 5-HT receptors since the binding of the β-adrenergic radioligand, [3H]dihydroalprenolol, was not affected by clonazepam. A second benzodiazepine, diazepam, did not affect the binding of [3H]5-HT at doses of 30 mg/kg per day. The latter data suggest that the effects of benzodiazepines on serotonin 5-HT1 receptors are unique to clonazepam.