We annually have ours down by the night of the 30th, so we can celebrate New Year's Eve as late as possible and not worry about looking like the dorks that forgot Christmas was already over a week previous... it makes for a relaxing New Year's Day not having to get out any ladders, step stools or boxes. Never let the New Year come in on last year's decorations, we always say. We did that one year... it stayed up way too long (albeit due to extenuating circumstance involving unexpected surgery complications). We only put ours up in the second week of December this year though due to uncertain economy and a desire to keep the bills low.
it is NOT uncool to leave Christmas lights up through New Year's; New Year's is still part of the festivities. Actually Christmas lasts through Epiphany, so y'all still ought to have your stuff up. Lots of people even leave it a little longer. It just means you're not through celebrating. At least allow for twelve days of Christmas.