I see a lot of Jeepers run the first couple feet of their winch line out to a mounting point on their bumper.

If you have synthetic line, would the exposure to dust, sand, ice, etc. take a toll on the fibers and reduce it's strength?

 

I wouldn't think it would be any different than having the line on the spool, I'm sure somebody will have some actually experience though....

 

Dust, dirt and ice wont harm it, but. Sunlight had a very slow but degrading effect. Also if you have a habit of nosing up to obstacles to see if you have clearance it doesn't like impacts with rocks etc so it may be better to keep it pulled into the fairlead.
Been using synthetic rope for years, used sensibly it fantastic. Main reason for breaking damaging it is sharp edges such as rocks and also violent shock loading. Its stronger than steel for a given thickness, but there is very little stretch so shock loading breaks it.
Nick