If you have a canister filter and your output nozzles are well under the surface like mine ... it doesn't produce any surface agitation. I run two air-stones for surface agitation which will help oxygen exchange.
You can add an air-stone whenever you want really .... I just added my second over the weekend. Plus, IMO, an air-stone can act as a short term back-up .., filters get clogged, impellers break, motors burn out ... etc.