As others have said there is no need to use engine flush if you already use good oils.
Synthetics and diesel engine oils are also great in cleaning sludge and deposits. You can use them in place of engine flush OR if you are using them then there is no need to flush.
DO NOT use flush/synthetic/diesel oils if your engine condition is not very good or you have been poor or old api rating engine oils.
Let me give you my example. I have a 1998 car and we have been using good (mineral) oils in it from the beginning. A few years back we shifted to synthetics/semi synthetic and there still is no problem. We also bought two different car 05&06 used models(around 60k km on the odo) at different times and shifted them to synthetics. After 1-2 oil changes they started consuming oil like crazy. There were two things that came in mind, either the cars were originally consuming oil but seller had put some additives in it to stop consumption or that the oil consumption began when the sludge was cleaned. I think it is the latter, given that most people still use obsolete API SG oils in their relatively newer cars, EVEN in cars like city, civic, corolla.