Is it necessary to have currency in an ideal centrally-planned economy? Under this condition, the price of goods are definitely meaningless.
How does the planner know that his central plan is the ideal one? The conventional story is that he's got an objective function that maximizes the sum of all the society's members' utility. He maximizes that sum by :


Setting the members' marginal utilities equal. 
Allocating inputs efficiently so the output lies on the production possibility frontier (Production Efficiency)
Choosing the mix out outputs to put to their most valuable purpose (Product-Mix Efficiency)
Allocating outputs efficiently such that no resources can be redeployed to make someone better off without making anyone worse off. 


How does he do this? He does this with prices. These may be shadow prices, relative prices without currency (barter), or they might be conventional prices quoted in the numeraire good. Explaining this is probably beyond the scope of a stackexchange article. This review may be of some use but the content in a microeconomics textbook would be similar. 

So the price of goods are not only not "definitely meaningless" but essential to the planner's activities. However, you do not need currency in an ideal centrally-planned economy. It is possible to write down models with prices but without currency (credit models) or even money (barter models) which nevertheless have central planners that can easily maximize welfare.