Depends what you flush with and the history of the fluid change, etc. in your radiator/ heater coil.

First level: the safe way is to use water and plan on leaving any crud in there that the water cannot remove.

Second level would be to use a low-aggressive flush in a bottle.

Third level, and most risky, is to use a two-part cleaner that employs more aggressive acid and caustic cleaners. You could clean a hole right in your radiator if you do that. The alternate view is that it would be a good thing to find now, and not in July on the freeway somewhere.

Good luck...