In a ClientBindind scenario, please use ClientBindingSettings ServiceEvents Selecting to add custom filter. This way the filter argument will be used during the data selecting (Paging will be taken into account)

Using the ClientBinding_Paging.aspx sample, add ServiceEvents Selecting. WebGrid Definition:

<ClientBindingSettings DataLoadMode="PagedData" DataSourceType="WebService" ItemTypeName="Bug"
    ServiceUrl="WebService.asmx">
    <ServiceMethods BatchUpdateMethod="UpdateBugs" SelectMethod="GetPagedBugs" />
    <ServiceEvents Selecting="WebGrid1_DataSelecting" />
</ClientBindingSettings>

 Here is the event handler snippet, all filter action will filter the data which has Target value similar to fut or bug:

function WebGrid1_DataSelecting(gridObj, selectArgs) {
    if (selectArgs.FilterExpression != "")
        selectArgs.FilterExpression = "[Target] like 'fut' or [Target] like 'bug'";
}