If you do not bring your nc flux to soldering temperature [~220*C], it is very dangerous from a product reliability standpoint to leave that material on your board.
Consider: * TP-1115 - Selection And Implemetation Strategy For A Low-Residue, No-Clean Process * Your flux supplier would be pleased to help bring you up to speed * Background found by searching the fine SMTnet Archives
very difficult to say that all no cleans must be cleaned for reliability. Cleaning in its own right causes many problems i.e leaving wax residues and activators from incorrectly cleaned no clean pastes, terrible to get off. Decent no clean liquid fluxes should completely volatise away so I always recommend if possible not to clean but if visible white powder(weak benign organic acid) is left then process the boards through a reflow oven at glue profile temps and this should volatise all organics off and not interfere with the rosins from the paste. Worth a try and it does work well.