What exactly are you trying to accomplish?
Is this a new installation that has never been run before?
Is this a check for maintenance after xxx hours of running?
When checking the insulation resistance of any system, you must first understand what you are checking and why. 5 megohms of resistance might be OK, but for a new installation, I would expect more. Also, I would like to see a Polarization Index done on each system.
If this is a system that is running, then why are you checking it? If it was bad, the breaker would have tripped a long time ago. Checking a system after running will only yeild good results if you have a baseline data. Without that, you are wasting your time.
If you truly feel any voltage is OK to test with, then use a Fluke and put it on the ohms scale. Anythnig more and you need to disconnect the softstart and cables and only check the motor.
Also, if you don't like the advice here and want to argue when good sound advice is given, then I would suggest that you don't waste your or our time and not even ask the question, especially with all your experience.
BTW, the term Megger is a trademarked name by the Megger company, formally James G. Biddle Co. and AVO International. The proper terminology for what you are doing is Insulation Resistance Testing.