It is not necessary to have a separate Workflow Server. However a lot of this depends on the amount of traffic you are going to have. If you have some lengthy workflows with a lot of users it may be best to have it separate. I am sure there are some guide lines here on the site that would give you more detail on if it should be separate or not. I have one location with 10 users but the workflows they had were quite extensive with several database lookups. They benefited having the workflow server separate from the Laserfiche server.