First, what is an official attitude, and did one exist in Williamite England?
In modern times, governments try to influence public opinion by the use of spokesmen in democracies, and propaganda in dictatorships. They have a 'line' on this or that issue which they repeat, hoping to make it catch on. I suppose this is what you'd call an official attitude. Some countries go to the lengths of writing an official version of history which no one is allowed to deviate from, but most democracies don't go that far.
In the 17th and 18th century, both monarchs and proto-dictators like Cromwell did a fairly poor job of controlling public opinion in this way. Public opinion was anywise a new invention: the public had no opinion until newspapers were invented and told the public what their opinion was.
The official attitude was mostly expressed negatively, through the suppression of material believed to be subversive. Pre-publication censorship persisted until 1695. Contra the comments to the question above, free speech was not yet a secure principle. British constitutional history, unlike American, doesn't have many bright lines. The Glorious revolution had increased religious freedom, but less so political. Even after the licencing act lapsed, it was still possible to get arrested for something which displeased powerful people, through the sedition or libel laws.
William and Mary did not use this power to suppress praise of Cromwell. Although they didn't want to associate themselves with a regicide, there were obvious parallels between the rebellions against Charles I and James II. Both were Stuarts, both were friendly towards Catholics, both tried to do without parliament.
In his own time and after, Cromwell has always been an ambiguous figure among republicans. He was England's only identifiable leader who was not a king. However, he took on kingly airs such as a sceptre and royal sounding title, and suppressed attempts to establish a democratic state. Moreover, after the chaos and bloodshed of the civil war, people no matter what their political beliefs were very determined that the peace, order and prosperity of the Restoration period should be preserved.
After the Stuarts' Restoration in 1660, Cromwell's name had been traduced. Even his dead body was defiled. If the House of Orange had joined in with that, they basically would have been admitting that the deposed Stuarts were the rightful rulers of England. Likewise if they had robustly defended Cromwell, they would have revealed themselves as lawless usurpers, just like him. It was best to just gloss over him as much as possible. If there had been a large number of works praising Cromwell, William and Mary might have been forced to reckon with Cromwell and make a decision one way or the other, but this didn't happen.
A modern analogue is the pacto oviedo in 20th century Spain, which means the pact of forgetfulness. After the return of King Juan Carlos, Spain agreed not to pursue claims of revenge or justice against either side in the civil war, and instead to just put the whole thing to bed. A similar thing happened in contemporary England. There were a few diehard republicans, but they were left unmolested because they had no prospect of achieving a return to Cromwell's system, or to anything still more radical.
This may sound like William and Mary were just burying their heads in the sand. If they were, it worked. Pressure for reform remained, but no serious attempt to establish a republic was ever made again.
