You SHOULD: 

Wash vinyl graphics with plain soap, water, or any car wash detergent

Keep high pressure nozzles at least 1.5 feet from the edge of the graphics.  High pressure spray can cause the edge of graphics to peel.

Test any cleaning solution on a small section of the vinyl graphic before using

Be careful when using a snow scraper on vinyl window lettering. 

You Should NOT: 

Don't let gasoline or other fuels drip or stay on your vinyl graphics for any length of time.  If a spill occurs, wipe off and rinse with water immediately.

Don't apply paint or clear coat over the vinyl graphics

Don't apply wax over graphics, especially if the wax contains any petroleum distillates.  Wax that has dried along the edge of a graphic can be removed by softening it with rubbing alcohol and using cotton swabs.