The only time I would use a cleaner/flush is when there has been a lot of rust or deposits in the system...otherwise I don't think it necessary.
To truly get all the old coolant out of the system, take out the engine block drains as well...they are two small hex or square headed plugs on either side of the block below the heads. While your at it, go ahead and replace the thermostat and any hoses that look questionable.