Good question.  I've never felt the urge, regardless of how I've felt about any given candidate.  I'm just glad I've never had a personal friend run for office as not putting up a sign for them would be awkward. 

I suspect signs still have some influence on local races, but their influence significantly decreases for county, state, and national races.  I doubt they have any influence on national races.  Similarly, does anyone even glance at those large format political postcards that fill our mailboxes this time of year?  I'm at the point where I can bin them just by feel.