I like to bracket my shutter speeds when doing such shots. The problem is that I only have a 9-stop and 3-stop ND at present, so I'm limited to small changes in ISO or aperture, with the associated drawbacks. For example, if a 3-stop ND doesn't produce quite enough blur, but 9-stop produces too much, I might leave the 9-stop on, use two stops higher ISO and 1-stop wider aperture (net result of about a 6-stop ND).
I've read that variable ND filters range from pure crap to tolerable, but I haven't chanced the money on any of the recommended ones.