We would always recommend varnishing a finished painting that is going to be hung or stored without glass. Not only does it provide a protective layer to your artwork it also brings up the lustre of the colours and evens out any dull or shiny patches, unifying and protecting the paintings surface.
It is recommended that an oil painting is left for between 6 months to 1 year to dry before varnishing (it will not continue to dry under a varnish layer), and if in doubt use a Retouching or Exhibition Varnish that is removable and can be used on pieces that may need further work done on them.
You have the choice of how to apply your varnish layer - either the traditional 'paint on' liquid varnishes, a beeswax form which is applied more like a polish, or the aerosol options which are quick to apply and to dry. Most are available in either gloss, matt or satin finishes.