Late Fall and Winter (through Feb/March), DEFINITELY (for Maryland, DC, VA, & PA area). Bikes always have a good drop in price in the cold months, especially from private sellers. And too, if you're looking to buy a brand new bike (which I don't recommend for your first bike), most of the dealers are having sales to get rid of the rest of their current year stock, as next year's stock has already started unloading.
For private sellers, especially in this area, people are always upgrading their bikes. As a result, they ride out the season, then sell their bikes cheap when it gets cold, so they can buy their "upgrades" cheap as well. I would assume that's the case in most areas, but I've only been watching the market here in this region. 8)