Forecasts are all over the place ahead of the media company’s May 3 report on its performance in the first three months of this year. The question is whether the strengthening ad market will outweigh the declining ratings at two of Viacom’s most important networks: Nickelodeon and MTV. Miller Tabak analyst David Joyce joined the skeptics this morning. He downgraded Viacom to “neutral” and reduced his price target to $51 from $56, in part because he believes Viacom’s ratings problems make it “the most-at-risk programmer for the upcoming earnings season.” Joyce now expects Viacom to report that domestic ad sales fell 3.4% last quarter vs the same period in 2011 — a change from his previous prediction of a 2.5% gain. But Susquehanna Financial Group analyst Vasily Karasyov reached a different conclusion on Tuesday. He stuck by his “positive” rating for the company while projecting a 1% increase for U.S. ad sales in the March quarter. He acknowledges the soft ratings, but says that the overall strength in the marketplace could show that early 2012 was “the turning point for US advertising revenue growth at Viacom networks” justifying his $58 price target for the stock. Bernstein Research’s Todd Juenger says he has a different concern: He fears that Viacom will spend too much on programming to help turn ratings around. Beyond what’s happening in advertising, he wants to know which of Viacom’s networks are “suffering from a persistent decline, versus a random blip.” Viacom shares closed yesterday at $46.40, and are down 2% over the last 12 months.